How do I get my cat to gain weight?

HELP!! I am a 10 year old and i have 2 cats one Thelma and the other hunter that is in serious trouble,shes almost 1 and she used to be 6 pounds now shes five! she eats and eats but never gains any thing shes also allergic to a lot of stuff like chicken beef corn beef corn,so she can only eat special foods she has all her vaccines and has been checked for hypothyroidism and overactive hypothyroidism but nothing came up please help! you can even feel her back bone and ribs you can even count them.