I wanted to add one more note.
If your ferrets are pooping beside or very close to the litter boxes then i believe that means they know they should poop inside the litter box but there must be something wrong with it preventing them from doing so. That Super Pet Hi-Bac Rectangle Litter Pan is the very best litter pan that i know of. You can buy them from Petco, Petsmart, or the ferret store. I bought mine from Petco, they are about 10 dollars and very worth it.
Click on Litter & Litter Pans. It should be near the bottom. There are different types of Super Pets but i recommend the rectangle one.
So if you have that litter box then it must be the litter.
1) What was the ferrets using as litter before you got them? If they are not kits and its different, then you might want to try mixing it with their old litter. When i first got my kits they were use to yesterday's news. I switched them as soon as they got home but they were kits so everything fell into place.
2) Your litter type sounds fine to me. So i know its not that it is a bad litter choice.
3) If you have more than one ferret, then one ferret might be sick and the poop is turning the other ferret away? If only one ferret then its not this problem. Otherwise just keep an eye on the poop and see if its any different.
4) At least once a week to every other week you should empty out all of the litter and replace it with new litter.After a while the smell of urine can build up.
5) Last but not least scooping. Which you said you did so i don't think not scooping enough is a problem.
I suggest just going through that list and making sure nothing is different or wrong with your pan or litter. Did you change anything when your ferrets starting pooping on the sides?