Why do my cats keep throwing up?

So I have 3 cats, all aged between 10 and 13. They will usually vomit after eating normal cat food in jelly or supermeat. We've tried changing from Felix to Whiskers, to GoCat e.t.c but it makes no difference. We've also tried taking them off meat and just giving them biscuits, but they don't eat them and they all lost weight rapidly. What is wrong with them?