If you are turning by hand, then when it happens turn the handle backwards one revolution. That may help clear it (some grain may spill out the sides of the rollers, hopefully the stone will too). I have found that too small a gap will prevent the grain from feeding into the rollers. In that case, the rollers spin freely for me. If everything completely stops - both rollers, the handle, then in my experience that's always been a stone. As samc suggested, a full cleaning and inspection is always a good idea. non-toxic silicone lube (a tiny dab on the ends) works better for me than the manufacturer's recommendation of vegetable oil.