I have experience dealing with hard keepers, let me give you my 2 cents worth.
Have her teeth checked.
Make sure she's wormed regularly.
Make sure she has access to good coastal hay 24/7. I keep round bales out at all times and when they are in a stall on acount of bad weather, I make sure they have hay to munch on the entire time. I don't put round bales out in summer, I have plenty of good pasture.
I find that if I do all of these things, grain is unnecessary. I do feed my horses half a scoop of crimped oats once a day to mix with their supplements. Tuffy gets Farrier's Formula because of his crappy feet and they all get Strongid C.
I think you can tell by Tuffy's pic that my system is working pretty well. Winnie the Palomino was losing weight so I just recently had her teeth done, wormed her good and I did bump her grain up to a whole scoop, instead of a half.
If all of that doesn't work I'll have to move her to another pasture since that would probably mean the other horses aren't letting her at the round bale. I don't think that's the case, I have seen her eating with the other horses.
Nothing to do with this but it seems her winter coat came in overnight too. I thought at first she was just matted but it's just thick, coarse hair.