Hi Lassie,
i had the same problem with my old tb, he was 25yrs old. now he lost weight within a couple of weeks, so i got his teeth done. turned out he was gaining some, but still very slowly. even though he got his teeth done he was still dropping a whole lot. so since i didn't want him to waste my expensive feed, i ended up soaking it in water over night. he would get
1 coffee can (reg size) of co-op senior feed, fat pellets (hi fat 20), cob, beet bulp and then , 1 cup of corn oil, 2 scoops cool calories and some other supplements for his joints. now i know it sounds like a whole lot and i kinda agree, but he was worth it. now once i started soaking it over night and he could just suck it up and munch on it, he gained weight like it was going out of style. and it was always hard to keep weight on him. now i'm not saying it was everything together, since it all depends on the horse, but i think because it was more of a soup he was able to retain all the noutriens better than dropping half of it.
so all i could suggest is, if you're giving your horse beet bulp soak the rest of your food in water as well and see if your horse will gain weight. i liked cool calories as a weight supplement as well, so you could consider that.
i know trickie liked making a mess out of his food in the end, but he ate it all. i mean it ended up in his mane and ears. he did pass on since his liver failed over the winter but in the last yr and a half when we started soaking his food he looked never better. no one believed he was 25.
good luck with the feeding and i hope your horse is going to gain some weight!