Dim Sum seems to be the same way with his pad as well. I've been trying to get him to go on the pad's. So far he's been trained to go outside.
I have a feeling he won't go on the pads because he thinks going in the house is not allowed at all. Especially when i'm standing there watching him..
Don't know what to say. Maybe just stick with it.
Maybe changing pads? See if that helps.