Potty training my guinea pig?

Well, I have two female guinea pigs who are recently being potty trained. The older female was born on October 16th, and we purchased her in late November or early December. She had babies on new years eve and we kept one who is now four weeks old. They both have been going to the bathroom on shredded paper for a while. I recently changed to fleece, and put a guinea pig litter box in the corner with shredded paper in it, along with some peed on pieces of paper, and some of their poop in it too, so they know that's where they need to go to the bathroom. The mother female always go's pee in it but rarely ever poops in it, she just poops all over the fleece! The baby female guinea pig who is four weeks old, sometimes pees on the fleece, and sometimes poops on it too! I've had the fleece in their cage for two days now and have only found two pee incidents. One done by the baby female, and the other one done by the Mother. I don't mind the poop so much because its easy to scoop up. But if it's possible, how can I stop the little pee incidents from happening every so often, and if possible to stop the poop too. Should I switch back to shredded paper for a little longer?