hmmm that's a toughie. Are they not in their own room or anything which you could confine them to, just for a couple of days and maybe build it back up from there? Perhaps they don't like the litter you use? My bun didn't like the wood pellets and started peeing in a different corner when I first introduced them, but now I cover them with a thin layer of shavings and he's happy as larry.
They can be right little fusspots can't they? :roll: I'm going to try that megazorb that everyone keeps talking about once I've used my current litter up as I think it's safer and less messy than shavings AND cheaper
Is it a cotton-style sheet? perhaps try cutting a section off and placing it into their litter box until they get the idea!
Sorry if not much help, I'm just bouncing off some ideas! :?