This happens pretty often with all different brands of powerheads and pumps. Often they will fix themselves within a short amount of time, but if not then you just have to take them apart and put it back together again, cleaning anything necessary. The problem is that the magnet attached to the impeller isn't spinning in sink with the electromagnets inside the sealed part of the powerhead. I have found that a quick disassembly and reassembly will fix it most every time.