Low pressure

Looking for some help. I have a cottage shallow well system that's been running fine for several years. The conrol setting is on at 30 psi - off at 50.
When I turned the system on this year, the pressure will not build above 28 to 30 psi - so the pump won't shut off unless I turn it off. I've checked for leaks - there are none. I swapped in an old pump - same result. The lift is about 20 feet and has not changed. Pump is 1/2 hp. I'm considering a new 3.4 hp pump - but I'm not sure this will solve it.
Any suggestions would be appreciated. This problem is driving me nuts! Thanks.
Fred

It's hard for me to believe you have two pumps with plugged jets, but that's what the 28 psi usually means. A plugged jet. It will be either in the pump or bolted on the front. The nozzle will have a piece of scale stuck in the opening. The nozzle has the smallest hole and the venturi has the largest.

bob...

Bob - Thanks very much.

I'll give it a try - probably next weekend.
Fred