Originally Posted by Kayty
Firs step - don't blame your horse.
But also consider what being hollow means: it's a sort of flinching away from your weight, which MIGHT mean he's sore. Unfortunately, the very act of sinking away can MAKE a horse sore too.
So the more you work on it, if he keeps trying to avoid discomfort by going hollow, the worse he's going to feel feel afterwards.
I would suggest giving him a little time off, then starting again in a long, low frame, let him stretch out. And keep checking to see if his back is sore. (Some horses are very fussy about the tiniest discomfort, so it might not be much, but there you go, you still have to deal with it.)