I too am having refusal problems with certain dates of Wellness....and a variety of flavors.  Oct 10, Jan 11. Feb 11.Can size makes no difference either.  For example I have 3 cases of Chicken and Beef an absolute fave in this house.  1 case of 3 oz 1 case of 5.5 and 1 case of 12.5 all with dates of 29 Jan 11.  Cats will not eat these at all.  March 11 5.5 oz Chicken and Beef were fed this morning and cats ate with gusto.

Wellness has been the primary feed in this house.

I have also noticed refusals of NV Instinct again certain flavors and I am assuming certain dates.
One of my problems is that I do not know how to read the codes on the NV Instinct cans.

Perhaps someone can guide me.

I am presenting my information too, as my cats are not picky and following their refusals, I try this on the ferals outside and they too, refuse the food.
I do not feed any dry food at all and I do not free feed in my household.

I sometimes rotate with 2 other brands and have never experienced a problem with those. 
ZiwiPeak..........Lamb and sometimes Venison
Weruva........Paw Lickin Chicken only

One thing to keep in mind is that Wellness did change their ingredient list starting with I think the December 2010 date lots.  Garlic was removed and some of the other ingredients were shifted around on the label.  This could be enough to account for problems with many cats and could account for a lot of what is seen for the date lots around Dec 2010-Jan/Feb 2011.  I have one cat with IBD that eats Wellness and I transitioned to the January date lot this month...I took 1.5 weeks to do it, mixing in the old lot and the new.  If any of you have cats that have problems as you jump from one lot to another, you might want to try mixing too.  I do this for any of the foods I feed my boys as even my non-IBD cat has had issues with transitioning.  This would certainly help rule out variation as the problem.   When I mentioned it to my vet a while back she said that is a very real issue they see with foods from any company, is that small variations from one lot to another can cause problems.

Jenny

jenny thx for info on ingredient changes. Wish I still had cans of the grain free that I was feeding my dog but she refused to eat it after awhile so took it back so nothing to compare it with for old and new ingredient list. So do not know how much the canned dog food changed and maybe thats why she wouldn't touch it anymore.

Like 3cat said maybe the labels are being used up and new ones not yet on the product being produced with the new best by?

That's a very good point about changing slowly to new formulas jenny.  A little odd to have to transition your cat from Wellness chicken to Wellness chicken, and they weren't exactly clear when the changes would be happening. 

Looks like all of us feeding commercial pet foods will be dealing with this issue for one reason or another for the forseeable future.  There isn't one company that hasn't gone through formula changes in the past year that I can think of.

It would be great if the PF companies would post on their websites exactly what lots were changing and when so we could transition for the pets that need a slow switch, but I guess that would be asking too much.

Well, I discovered after transitioning two of my cats to canned food in spring 2007, that both of them were sensitive to changes between date lots of food.  Bo was the worst - he is the one that was eventually diagnosed with IBD.  Sometimes the problems were very easy to miss - he would just be a little off one day, maybe vomit once maybe not at all, but just didn't seem quite right.  But after keeping a diary I started to recognize a pattern....interestingly according to the diary I didn't have problems when I switched from one lot of turkey to another, but from Wellness chicken to chicken I almost always did.  That is one reason I only use the turkey now. Actually according to my diary there is some evidence that Bo might have issues with chicken as a protein source (based on trials with other foods too), but for some reason he has always done really good with the Wellness turkey (which contains chicken) so after talking to the vet we've elected to keep him on Wellness Turkey & EVO venison. He's been stable since about end of February now.

Wilson does not have IBD.  I had been using the same date lot of turkey & salmon for almost 5 months for both Bo and Wilson and purchased a new lot in December.  I remember opening the can and thinking 'oh, no - it looks completely different'....But Bo was already having IBD problems so wasn't eating that variety at the time so I didn't give it a second thought.  Wilson has always had a tough stomach.  But not this time...again no vomit, but he lay around and didn't move all day and didn't want to eat dinner.  He growled when you bothered him.  He hasn't touched Wellness since.

But I did talk to my vet and she said not to underestimate the changes from just one date lot to another.  So, since Bo has IBD and Wilson has now gone off both Wellness, Fancy Feast, and briefly off venison (I blame nature's variety for the venison...don't think he can eat it)....I've just learned to be careful.  I now always blend for 4-5 days for both of them.  And since I knew the ingredients for Wellness changed I did it over 1.5 weeks for Bo for turkey.

And I'm sure not all kitties are as sensitive as Bo & Wilson.....I somewhat blame the Royal Canin Weight Control prescription kibble my three cats ate for years.  Cougar grew up on the food and was diagnosed with IBD in 2004 when he was 4.....Four years later Bo develops IBD.  And although Wilson does not have IBD, he has some sensitivities and I also believe may have some food allergies.... I would absolutely never recommend that food to anyone, ever.......

Cougar now eats Hills I/D (yeah, not a great choice but he can't eat canned foods),  Bo eats his EVO/Wellness, and Wilson eats a variety of canned foods with a small amount of kibble.

I don't think it is just changing ingredient lists always, although in this case for the December 2010-Feb 2011 issues it may very well be....I think just little differences between lots.  If you look on the Wellness thread there are all sorts of comments on how the food looks different from one batch to another.  Some more firm, some more jelly, etc....Those little differences may be just enough to bother some cats.  I think on another thread a while back we talked about quality control, which may be an issue.  Would be interesting to know the real reasons for the variations.  Is it the use of natural ingredients that vary from batch to batch, is it a quality control issue....or maybe both.