I have a cottage system where we pump from a lake. Used to get 30 - 40 psi now only 25 - 30 and the pump won't shut off if I adjust the pressure switch higher.
Have installed new impeller kit in the pump and replaced foot valve and pipe connections with Hanflo plastic fittings. If pressure switch is set low, it pumps up to that 25 - 30 psi then shuts off and holds pressure, so doesn't seem to leak, but just can't get pump to reach higher pressure and shut off and hold.
Could it be the pressure tank?
Other suggestions?

Assuming you have a bladder tank, there is a small air fitting towards the top, like on a car tire. Depress the stem for a few seconds. If water comes out, then the tank is bad.

If water does not come out, then set the tank pressure. With the pump off, drain water from a low level faucet until pressure drops to zero. Set the pressure at the top of the tank to 2# BELOW the pressure point at which the pump turns on.

Generally, in a situation like yours where you are pumping from a lake and the water level is pretty constant, the source of the problem is the pump. How old is yours?

Assuming you have a bladder tank, there is a small air fitting towards the top, like on a car tire. Depress the stem for a few seconds. If water comes out, then the tank is bad.

If water does not come out, then set the tank pressure. With the pump off, drain water from a low level faucet until pressure drops to zero. Set the pressure at the top of the tank to 2# BELOW the pressure point at which the pump turns on.

Generally, in a situation like yours where you are pumping from a lake and the water level is pretty constant, the source of the problem is the pump. How old is yours?

Pump is about 5 years old, but as I said, I have overhauled it with a kit.
I did try pumping more air into the tank, but wasn't sure what to set it to, so I stopped. It was about 20 psi and I think it should probably be about 30 psi.
I could do as you say to test for blown bladder and if OK pump up to 30 psi then run pump. It should shut off at 25 - 30 psi then I could crank up the shutoff pressure a little and se if I can get to 40 or so.
What do you think?

Just bear in mind that the tank pressure should be 2# BELOW the cut in point for the pump. So, for example, if your pump cuts on at 20#, then you would set the tank pressure to 18#. However, if the tank is good, then that is probably not going to be your problem. Your problem is very basic. The pump does not seem to be able to get past 30#. Unless the lake level has dropped fairly significantly, or there is a leak in a pipe fitting somewhere (unlikely, since your pump seems to be holding prime), then the attention gets turned to the pump. Granted, you replaced some parts, but it still comes back to this. The pump cannot seem to get over 30#.

Look at it this way. Only the pump can BUILD pressure in a system. The tank simply STORES water under pressure by means of the pump putting water into the tank which compresses the air in the tank.