Help mare won't gain weight!!?

I have a four year old who I have on weight builder, hard keeper, pro-bios, red cell,gets wormed at least every two months, and gets grain with timothy hay pellets, along with a whole bale of hay a day but she won't gain weight!! I need help!! I'm scared for her! I had her checked before and the vet told me she had worms and prescribed what I needed to do and I did it but she's still underweight. I can see ribs!! Please help I will try ANYTHING!!!