I went back and tried to sand away any under cuts
I don't think it is that as I put a small pug in the middle where there are no real hard edges and with the air pressure it still did not pop out. I am thinking that it has something to do with the clay turning into a kind of mud and forming water adhesion, suction, that the small air bubble aren't overcoming.
I tried to dry it off a bit and apply slightly drier clay (which is harder to push into the design). But it actually released better -- haltingly-- after about 8 seconds. But look at the clay film that it leaves behind on the ceramic mold. This suggests to me that something is wrong with adhesion to the ceramical for some reason.
Any thoughts?