I just recently bought two ferrets and what's odd is that my little girl is potty trained but my boy isn't. He poops EVERYWHERE and I have to clean the cage constantly. I took out all the levels because the pet store I bought them from recommended that and it still doesn't work. I will add a picture of how their cage is set up. Any tips will be really appreciated. In the picture I just finished cleaning it. I use vinegar and water mixture. 

Here is some suggestions:

I like to use a high back litter box or a cat rectangle litter box.  Ferrets like to get their whole body into the litter box.

Cover all your corners in the cage.  (water bowl / food bowl or toy)

Leave some poop in the litter box so that the little one knows that is the potty area.

I hope this helps

Covering all the corners other than the litterbox one will be your saving grace....of all thenaccidents my ferret had when i first got him, ALL of them were in corners...put a blanket, a food bowl, or a toy in the corners...its highly unlikely he will poop on top of them. Also, if you hsvent done this, put a bit of his accident into the litter box, it might help him realizse "hey, this is where to go" you could also try a differemt litter, maybe hes picky and doesnt like the feel of it on his toes? Stay persistant and consistant!!