http://www.depcopump.com/datasheets/flotec/FP313-EN.pdf
Maybe your check valve is letting water go back down the well.
Troubleshooting steps pg 13 in manual list several things.
Check air valve for leaks.
Flowtec has a customer service number:
1-800-365-6832
The manual says: "Your new pump has a 30/50 PSI
switch, so adjust the tank pre-charge pressure to 28 PSI.:
But the manual does not say or show how to adjust pre-charge pressure.
The 30/50PSI means the pump turns on when pressure falls to 30, and then
runs until pressure rises to to 50. So the switch is preset and not
adjustable.