I have a submerged well pump with a 20/40 switch. The cutoff pressure is fine but the system will not cut in untill the pressure drops to almost zero. I have replaced the pressure switch and the problem persists. I measured the air pressure in the tank and it was below 8 psi. I increased the tank pressure to the recommend 18 psi and no cutin occured at all. I then decreased the pressure to the previous level of 8 psi and it resumed cutin after pressure near zero as before. What do I check next.