Solid (especially plastic or metal) shelves often allow the urine to puddle and they will sit in it, especially if they do it in their sleep. If you have solid shelves, you might want to angle them very slightly to the center of the cage.
Supply plenty of bedding and clean weekly, or more if needed. It is just as important to clean the shelves (even the wire ones) on a weekly basis as it is to change the bedding.
Sometime is it the
environment and other times it is a
learned bad habit ... if the mother did not keep herself clean constantly, the kits will often follow mom's bad example. For this reason, a chin that stays stained will not be put into breeding by me and becomes a strict "pet only" chin.
Jo Ann