I recently replaced my pressure gauge and fixed a hydrant. After turning the well back on I have low pressure to the building. Pressure switch shuts off at 42 psi and psi slowly falls, low pressure to building (good for 3 seconds then falls to almost nothing), pump kicks on at 20 psi to fill tank back up. No signs of a leak. I was told my pump may have lost its prime but I thought submersible pumps were self priming. What could be the cause of my low water pressure to the building? Thanks.