What sorts of things have you tried with them? I find that different pets have different tastes, just like we do! Maybe one will like carrots but not cucumbers for example. I think you have to be careful with fruits and maybe carrots with dwarfs though because of the sugar content but hopefully someone will be along who knows better than I.
Just keep trying tiny bits of different things and sooner or later you might hit on something they like! I once had a chihuahua who would do anything for chickpeas