Poor Callie and poor you.
It's so frustrating, isn't it? When you can't exactly pinpoint but KNOW something is off your with baby.
Wellness never fared well with any of our dogs for some reason, either. I remember finding moldy looking stuff in a bag of Wellness my dads dogs had too a while back. Not to say it's a BAD food, I've known dogs who do well on it, but ours just didn't.
A few months ago, Jackson had an upset tummy and threw up 3-4 times throughout one weekend. It's really rare for him to be sick like that, he has always been able to switch foods easily, etc. But the two things I had done different were feeding the Rachel Ray canned food and feeding MilkBone dental treats. I went to open a new tub of the RR food (I had a box of 8 of them) and the food smelled NASTY, it was like rancid and just... not right. So obviously I threw it away along with the rest of the box. It was odd because all the other tubs of food 'smelled' fine and he HAD eaten the RR wet food in the past. But the only two things I could attribute it to was those, so we just stopped feeding both and never will again to be safe. I was prepared to take him to the vet but once we stopped these and he ate chicken/rice for two days, he got better.
Jackson's been back on Acana because it's easily accessible to us (the Farmina wasn't; only sold in a store far away and Chewy.com is always sold out of our sizes) and even though I was bummed with the ingredient additions of lots of lentils and peas, honestly, he just does best on the 'Singles' line - the duck and pork formulas. He seems to do best overall on really simple formulas, one or two meat sources, nothing terribly complex. I forget if you ever tried the Singles line with Callie.
Anyway I am rambling. It could very well be the food or it could be something else. RX food wouldn't be the end of the world if that's what it comes down to, even though they're so costly, I know.