We have an in-ground well pump connected to our sprinkler system. We went of vacation and when we returned, our grass was dead. The well is barely pumping a small amount of water through the system, but it won't push the heads up. I checked the relay and the wiring at the head of the well and everything seems fine.

I pulled the well pump up, dropped it in the pool and it pumped 18gpm with no issues (into a bucket). When I drop it back into the ground though, I get nothing through the sprinklers. What should I test/try next?

Edit: Pump is a FloWise P18S10, 1 horsepower 18gpm pump. It has worked fine for 5 years, but just quit while we were out of town.