When using larger molds such as the ones for belt buckles and paperweights, the resin will often stick and can be difficult to remove from the mold. As these molds are not flexible, you can't pop the resin piece out like you can with the smaller jewelry molds. I had this happen once when making a belt buckle and I almost threw the whole thing away out of frustration. I contacted the resin manufacturer and they gave me a very helpful tip that really works.
Turn the mold upside down so the resin part faces the table. Take a heat gun and move it all over the back and sides of the mold. Pick up the mold and slam it against the surface of the table a couple of times. The resin piece will inch it's way out of the mold. The heat from the heating tool constricts the resin which allows it to slide out of the mold. It really works!