Hypoallergenic foods are not guaranteed not to provoke an allergic reaction, they are just less likely to as the most common allergens are not present. Many have large numbers of ingredients so it can be really difficult to isolate a possible cause.
Are you leaving the fat/ skin on the chicken? That will provide essential saturated fats and will help boost the calorie content of what is being fed. Chicken leg meat is higher in taurine and fat than breast.
You might also try a little raw offal, my Noah will pick that out of his bowl first if it is bloody.
Ideally you would get chicken offal, but heart is difficult to find. The proteins in raw meat are different structurally to that in cooked meat so *hopefully* there won't be any reaction. Do only serve small quantities tho as liver especially is high in certain nutrients and you don't want an upset tum from a change of diet!
My last thought is has your vet advised you to give probiotics (friendly bacteria)? These are important for immune function, for absorbing nutrients from food - including B vitamins IIRC - and can easily be depleted where there is a digestive problem. It's not a cure obviously, just help with managing the situation. In humans omega-3 supplements (fish body oils) are effective in IBS/ IBD as they have anti-inflammatory properties; please check with your vet before giving this as it can have implications for certain medications and felines have different requirements as to fats than we do.
