You aren't feeding enough hay. Unless you have monster flakes, 6 a day just won't cut it. The best way to put weight on a horse is as much excellent hay as the horse can eat.
Definitely up the hay. If it takes her a long time to eat, section her off until she's totally done eating. It sounds like you feed them all out in the open, let this one eat her hay in peace where nobody else will interfere.
I'd also add hay cubes or pellets to the horses grain, and soak it all down to make a mash. Easier to eat.
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