Giving 3 ml of Baby Gasx (liquid Simethicone) will also really help make it easier on them to break up the gas bubbles if you are lucky and catch this early on, but don't wait too long. If bunnys bloated tummy doesn't feel softer and they don't perk up and start eating and pooping they probably need the fluids, pain and gut motility drugs. They usually hide their problem so long that but he time it's noticeable you need vet help. You can styringe some water or the gasx and try to see if their tongue and mouth look normal pink. If they are looking pale with almost a blue tint you should get to a vet asap. They are dehydrated and need subq fluid and immediate help! The sooner they get treatment the quicker they recover.
I've been part of rabbit rescue and fostering for over 15 years and unfortunately have been through quite a bit of this. Some bunnies seem more prone to it. Especially ones that aren't very good hay eaters.