A jet pump uses pressure to make more pressure. The bladder tank needs to have enough reserve so that when you draw water, there is still pressure available to the pump.
It sounds like one, two, or three problems. The precharge on the tank may be too high, not leaving enough reserve or the bladder may be busted. The ejector on the pump may be plugged. The line to the pressure switch or the diaphragm on it may have sand in it causing it to malfunction. The same sand can cause the pressure gauge to read wrong.