Impeller will not pump. Checklist of things done: two new impellers, checked pin to make sure there, inspect housing, made sure it is pointing in right direction, checked for obstructions above and below impeller, made sure gommit(?) fits securely on pipe leading to motor, only tested in lake with water level right below where it comes out. It does have a crack in one of the walls right below where metal plate sits below impeller would this make a difference. any suggestions will be highly apprecitated.