This happens as operators are drilling through sand or gravel. When the operator stops circulation to add the next section of drill pipe, the cuttings settle around the bit. If this occurs, operators should allow extra time (as much as 10 minutes) to flush the cuttings and loosen the stuck bit. If this does not resolve the problem, try thickening the mud with bentonite to assist in removing the cuttings. For more help read here.