I have almost all issues with my cats. First, they gained too much weight on dry food (Iams and Science Diet) so we adjusted their intake and measure fed them twice a day rather than free fed them. They lost a lot of weight this way too.
But, then one got really sick and it turns out she has urinary tract disease and the ash in the dry food causes issues- she used my brand new furniture as a lter box

. So, the switch to wet food was on.
I tried expensive stuff from the vet, and then Natural Balance and my cats would NOT eat

! After a week of not eating I threw a treat at them of Fancy Feast and they ate it. Feeling better, I tried the good stuff and the would still NOT eat

. The only wet food they will eat is the Fancy Feast Grilled in gravy- chicken, turkey or beef only. Basically only the stuff that looks like tuna.
How frustrating, the vet isn't happy about thier food choice, but it's either that or nothing because they just won't eat it.
Good Luck! PS, now they eat well and are skinny too!