Uncovered my pool to find that the water had dropped several feet from where I left it in the fall. My pool guy thought it was probably a leak in the liner. Told me to drain the pool, plug the main drain at the bottom of the deep end, and back fill the main line to see if there was a leak in the main drain. I went to drain the pool from the main drain, and the pump isn’t pulling water. Not an issue with the pump because I poured water into skimmer line and it flushed out just fine. Worried that the air lock on the main drain was broken and that there’s a crack in the line at the level where the water settled (about 6 ft below ground). This seems unlikely though as it would be pretty deep to have frozen, even in South Dakota winters. Any thoughts?