Ok, that is helpful to know. Here's what I usually suggest for these cats:
1. I like to use low doses of oral Prednisolone once the injection has had enough time to clear the system. I find it works better. This is a steroid, and I use it until we figure out the source of the problem.
2. Good quality flea prevention to rule out flea allergy - even if you're not seeing them!
3. Single protein food, preferably canned, but some cats will only eat dry. I like to stay away from fish and chicken. I really like the duck based diets for cats!
4. There are blood tests that can be done in cats to help determine environmental allergies, but they can be costly, and allergy injections sometimes help and sometimes don't.
5. Sometimes an antihistamine called Chlorpheniramine can help. You can get it over the counter at any pharmacy. In cats, we usually use 1/2 of a 4mg tablet twice daily.
6. Some of my clients find that using essential oils can help cats with allergic skin disease. There is a website called AnimalEO that makes one safe for cats called KittyBoost if you are interested.
Please post back with further questions!