With this pump it is common that the shaft begins to bind and even fully calcify to the magnet. This would cause low performance, start up issues and eventually pump failure if not maintained. What you want to look for is first check the shaft to make sure it removes from the magnet/impeller assembly and is in one piece. Check the magnet and needles on the impeller for damage, buildup and debris. Soak the pump and all the parts in a mix of vinegar and water. Also include a inspection and soaking of the venturi (air injection point) as well.