hi all I seem to have a sinkhole forming around my well casing and i don't know what is causing it???

Good morning all, I have a jet pump that I guessing is over 10 years old. It is a 20psi to 40 psi pump. It is a shallow well that is around 35 to 50 ft deep. It is at my camp so the pump does not get a whole lot of usage (1 or 2 weekends a month). I just bought this camp about 2 years ago and it is my first well so I'm new to pumps. My problem is that when I try to prime the pump and turn it on, the pump take a long time to get any pressure and I have to sit there holding the switch in the on position. When I do this, the pump eventually gets pressure up to 40 psi and shut off, as it should. From there, the pump loses pressure without using any water in the camp, doesn't turn back on at 20 psi, and drops all the way to 0. I checked the water level in the well and there is plenty of water. I am going up this weekend to work on it and was hoping to get some feedback on how to troubleshoot this issue. I was planning on checking the foot valve first. I did this once already about a year and a half ago and the foot valve was old but still in working order. Next, I was going to check the airpressure in the tank to see if the tank baldder may be bad. Any other suggestions on what I should check and how would be greatly appreciated. Thanks.