Litter training

I'm having a problem with Nibbler and his litter training. Since moving to the new flat Nibbler keeps pooping outside his litter tray. It's still in his hutch, just right next to the litter tray. Since giving him a dig box I've noticed him do it more, and I've started to notice some poops on his fleece in his pen outside his hutch and there was even a wee on the fleece the other day. I'm constantly picking it up and chucking it back in to his litter tray, but I don't know what to do to encourage him to go back to only using the tray. He's not avoiding it as he poops in his litter tray as well, he's just insisting on pooping outside his tray. Anyone know what I can do to get him back to only using the litter tray? I tend to over fill his hay rack and he pulls it down to the floor of his hutch and uses that as the base for his pooping. Wondering if I need to not fill it up as much, but the reason why I fill it quite full is Nibbler isn't a very big hay eater, and if there's not much in the rack I've noticed him ignore it completely.

I wish I could help but I'm still fighting the same war myself