It's probably not their poop you're smelling, but uneaten cecals (cecotropes). If they are overproducing cecals you may need to change their diet. Too many treats can cause overproduction of cecals. It could be that you need to cut down on the amount of pellets they're given. If you're feeding them vegetables, you may need to decrease the amount fed. The bottom line is, they probably need to eat more hay. Pellets and vegetables should make up, at most, 20% of their diet. If your rabbits are still young and growing, and you're feeding them alfalfa, it may be time to start mixing in Timothy based pellets and Timothy hay to wean them off the alfalfa.