I would continue to spot clean the cage often, at least a few times a day and put it into the litterbox. Usually that's enough for them to pick up on it but it does take a while for them to fully get the hang of it. Some catch on quick but some can take weeks. It just depends. If you are trying to think of a way to positively reinforce using the box then maybe you could give them a small treat when you see them use it? Something they can have multiple of like cereal or something. I've never tried it but I would imagine it would be helpful. you can't see them all the time when they are using it of course but you could at least catch them using it some of the time and they are smart enough to put 2 and 2 together.