I found the problem and I'm almost too embarrased to post it here. I had jAmpTitleBottom set to 0 believing this was meant to be the distance from the bottom. Instead it should have been the distance from the top in which case it should be 15. Obviously jamp ignores this value if it's too low when not used in a box.

Thanks for uploading your theme Egonz. I wouldn't have spotted my mistake otherwise.