Ferrets can be very particular about the type of litter and also the type of litter box...some like the corner boxes and others prefer a rectangle box so they have a choice of 4 corners *roll eyes*
If possible can you somehow block off the other corners in the cage, so that the only corner is in the litter box? What kind of litter was he used to?
The thing that worked the best for us was to catch her in the act of pooping in the litter box. When she did it, I praised her and gave her a squirt of ferretone. We had to clamp the box to the cage side so she wasn't able to get behind it to poop also. The corner litter box was not good for her, so I ended up buying a plastic rubbermaid box about the size of a shoe box and clamping it to the side of the cage. She apparently likes having the 4 corners to choose from for backing into. We use non-clumping cat litter. We also used something called ferret-off which we sprayed in the areas that she pooped that we did not want her to go in. I'm not sure how much that worked, but eventually now she never poops anywhere in her cage except in the litter box. She still leaves little presents in the corners of the house though when she has been out a long time.
I have been sent a Valentine
~Annie~ B.B.- cat, Toffee-ferret and RIP hedgehogs, Prickles, Pepper, Coconut and Nutmeg