Hi, OP. I am completely aware you can't diagnose/give treatment over the internet, but I'll try to present you my situation see if you have any suggestions
I found a stray cat in April 2017. She was so tiny (400g) and the vet estimated she was almost one. She had rhinotracheitis and conjunctivitis which were subsequently treated (v. Hard, we thought she was not gonna make it).
Basically as she was so tiny and almost dying we fed her wet food (royal canin instinctive) which she seemed to like. We then bought dry food but she has tiny teeth and she seems not to be able to chew properly so she doesn't really eat dry
She weighs 3 kg now, but she is a tiny cat with a tiny head and everyone has said this is the max weight she will reach.
Since then we gave her immunity treatments and her (apparently chronic throat sensitivity) hasn't returned. She's on and off re food, has days when she eats properly and days when she eats close to nothing. On the days she eats properly she eats 2 pouches and on the days she doesn't maybe 1.
Apparently she also has a mini problem with gingivitis which she had treatment for and her teeth will be soon cleaned (can this be a reasons for not eating?!)
I'll have to say we tried all the premium brands (better than rc) and she won't eat those either, she just seems fussy. Also, she drinks close to no water at all (I assume it's because of the wet food?!)
Please advise if you have the time and you managed to read all this. Thanks in advance for your time