You may have a tank leak on the air side of the bladder. It doesn't take much leak to be large enough to leak out at fast as you can pump with a bicycle pump when you get to 21 psi.
After you pump it up, immediately check the air pressure and then watch it for a few minutes. If you lose air in a short time, then the tank is leaking.
I chased a problem like that for a while and discovered the air leak at the welded fitting near the outlet on the air side of the bladder. It was not possible to see it when the tank was installed.
You might be able to check it with a little water in the tank. When there is water in the tank the water pressure gauge will read the air pressure. If the pressure drops with no water usage and pump off, that is another indication of an air leak in the tank.