I might suggest that you have a fecal test done. This will tell you if there is something going on with parasites.
My mare is somewhat of a "har keeper" She will sometimes have what I call "ploppy poop" or cow pies.
It comes and goes.
When she arrived she was pretty skinny and shortly after ariving to me she went to 60 days training.
Returned and had to have surgery. So it has been a battle for the weight issue.
What I have her on is 3 flakes grass hay morn and eve.
I give her about 3-4 pounds of Ultium and 5lbs of Rice Bran pellets.
She can not go out to turnout for grazing so when I can I hand graze.
For her nervous tummy I have started her on SmartPak's Succeed. It says that it takes 90 days to see a good result, but I have already noticed some difference.
I have heard the vets speak about Weight Builder as a good choice for getting weight on, but because my mare is a picky girl I have not tried it. I figure since I finally am seeing some results with what I am currently doing why change things....
I think you will have to find what your horse likes but something other than sweetfeed. That is to me the equiv- of junk food. But to each his own.
Different things work so there are many products out there.
LMF supersuppliment is another and they have many varieties to choose from.