My pony does that too but she's just being stubborn. My instructor put a tongue grid on her bridle, it's like an extra bit that points higher up her mouth, and it seems to do the trick.
I know Daisy reins are used to stop the pony putting its head down to graze. What I wanted to know was whether it could be used to prevent my pony from refusing at jumps, as she tends to put her head down on the ground. Or if not, what else could I use? Thanks!