I just got a Kitchenaid ice cream maker attatchment and tried it out for the first time. Everything seemed to go fine except that while the mixture was churning, the texture looked a bit lumpy. It wasn't silky smooth like I imagined it should be. When it was finished, I put it in an airtight container and set it in the freezer to "ripen." The flavor of it is good but the texture is off. It has tiny bits of ice crystals in the ice cream itself. What causes this?