if your not using disinfectant that might be why theres mould!
As although your cleaning well the invisible bacteria is still lurking
( i sound like a mr muscle advert or something?
)
i use
Johnsons Small Animal Disinfectant Spray - Pet Universe
which even has a little hammy on the bottle
which has lasted me for ages and although it makes cleaning out take a bit longer (you have to leave it for 15 minutes afterwards) it makes the cage smell really nice and fresh.
It makes it feel clean.