Peeing on Fleece

I have four beautiful 7 year old girls. About a year ago i moved them to two Ferret Nation cages. I have been having issues with messy butts ever since =/ As you know, the shelves are plastic and must be covered in fleece. I have heard of many people allowing their chins to pee on fleece and have no issue. For whatever reason, my girls get very dirty bottoms when they pee on fleece. Its mostly just my two bigger girls that have the issue. I cover most of the fleece with bedding so that they can pee on that instead, but they often times prefer to pee on the fleece. I am worried about their hygiene. Sometimes their butts become very wet. I use a layer of fleece, two layers of towels topped off by another layer of fleece. Am i missing something? Is there any way to deter them from peeing on the fleece?