Pouring too thick. If you are unable to pound the bubbles out of your mold, it is too thick. However, if you are using silicone molds, those are notorious for developing bubbles on the edges. They are easy enough to plane off if they bother you.
Are they big bubbles, or lots of tiny ones? Tiny air bubbles can be caused by stick blenders. I have a new blender that creates tons of tiny bubbles regardless of how much I "burp" it, whereas my old blender works great. If they are big ones, I agree with the others that if you can't bang your molds and get them to rise I would try pouring at a thinner trace.