Litter boxes: The Defiant Ferret

Question
I currently have three ferrets, having acquired the first one over three years ago. Ziggy, our firstborn, was on sale because he was a bit older and no one had yet purchased him. This was a bonus to us, as our first ferret, because we did not have to litter train him, or teach him not to bite. He was very receptive to our rules of pooping before playtime, and returning to the litter boxes in the cage to do so. A few months later, we got Mo because we thought Ziggy might get lonely while we are away during the day. Mo is a hefty ferret, and quite the glutton. He often has softer stools, and sometimes runny, but he shows no other symptoms and our vet didn't seem concerned about it. Two years ago we adopted Lily from someone who couldn't keep her anymore. She also was already trained in all aspects. Being a Psych Minor, I went hardcore with the classical conditioning to make sure all three ferrets knew that if they pooped in the litter box, they would either get a treat, or they would get to come out of the cage. This has worked fantastically for Ziggy and Lily, and they will even return to the litter boxes in the cage during playtime. Mo, on the other hand, is a different story. I understand that all of them will have the occasional accident, but Mo is defiant! When he is in the cage and has to poop, he will get up and look around the room to see if a human is watching. If no one is watching him, he will poop in the middle of the cage, right in front of the water dish, right by the ramps, on the blankets, wherever he is standing! He only uses the litter box if someone is watching because he KNOWS he will get a treat or get to come out of the cage. We even bought a bigger cage so we could fit more litter boxes inside, and have them on different levels. We scoop the litter boxes daily,and they are also strapped in place. If I am in the room and I see him trying to poop in the cage, I stop him and keep putting him in the litter boxes until he goes. Do you have any training suggestions for Mo? I feel so bad when I come home from school or work to see that he's gone right in front of the water dish and now the other ferrets have to try to get around it to get a drink.

Answer
As I am sure you can imagine, litter box issues are probably my most often asked about problems!  They can also be hard to "fix".  To help a little, the other ferrets I am sure don't mind stepping over or around the poo.  I had trouble training my ferrets to poop for treats as they would only poop if I was around to let them out or feed them a treat.  In most cases, more litter boxes is a great solution.  My experience tells me that early experience is the best predictor of future litter habits.  If a ferret lived with mom and siblings for a while, then they are more likely to use a litter box.  All you can really do is supervise when you are home, continue doing what you already are doing, and be patient.  I am guessing his early experience didn't have appropriate space or litter places and this makes it much harder.  You can remove as much as possible from the cage (blankets, etc) and use a water bottle, hammocks, etc to discourage the possibility of pooping in these areas.