Well Pump not building pressure

Hey everybody, I'm a journeyman plumber in the state of Montana, and I have a real head scratcher on my hands. I installed a well pump and pressure tank at one of my customer's houses a few days ago. I have been working on systems like this for ten years, so not my first rodeo. Everything that could go wrong went wrong in the process. The new pressure tank had a problem from the factory and had a leak, so I replaced it. But the pump itself is baffling me. The customer had an old Sears pump, 1/2 horse shallow well with 115v power supply connected to a 30/50 pressure switch. The reason for replacing these things is the pump is 20 yrs old, the pressure tank 12 yrs old and the pressure tank's bladder had failed. The customer figured why not replace the whole thing. I replaced the pressure tank and set the psi in the tank to 28 lbs, considering the 30/50 pressure switch. I then replaced the Sears pump with a Meyers 1/2 horse that was factory preset to 115v. The pump ran exactly one time. When I bled the pressure back down to get the air out, the pump would kick on and hum like the impellar was stuck, and then kick out on thermal overload. Whatever, I figured, faulty pump from the factory. Weirder things have happened. I take the pump back, but this time I buy a higher end A.O. Smith pump. Once again, 1/2 horse 115v. Both times they come with a new pressure switch, both of them being 30/50. I install this one, and it works. Bleed the air out, it turns on at 30, off at 50. No leaks, no drips, no errors. A day later, the customer calls me. The pump won't shut off. I run out to his house and find that the pump will only build about 38 lbs. of pressure in the system. Now I'm blown away. Why would it work for a day and then not be able to build the pressure? I put a gauge on the pump as it's running and it's producing 30 lbs. of pressure as it is running. I attempt to bleed any air out of the system, there isn't any. So I figure, maybe that pump is faulty too. It happens to be Thanksgiving, so no parts supply houses open, at least not for a reasonable amount of $. So I take his old pump, which was working fine, and install it to get him through the weekend till I can get a new pump. I install it and it works fine. This time, however, I set the cut out pressure a little lower, to 45 lbs., figuring that should get him through the weekend. Now he calls me this morning, a day later, and once again, the pump system will only produce about 33 lbs. of pressure and won't shut off.:madhell: