Pomeranian Housetraining Questions

I have a 4mo/old neutered male pom. I'm crate training him & he's good about not going in the crate. Generally no accidents, even overnight. I take him out often during the day. The problem I'm having is I have a small indoor pen (30"x30")for him to play in when he's not in his crate or outside. I am in the same room with him when he's in the pen but he has a lot of accidents. I feed/water him in this pen & take him outside before & after he's put in there.

So my question is, if he can wait up to 8hrs overnight, go potty outside right before he's put in there, why is he still having accidents? Am I doing something wrong? He's not in there that long before he seems to have to go either.