Update: I was burying my fence posts way too shallow for 3 years and my gate sagged to prove it
Last month my gate in the backyard started dragging on the concrete every time I opened it, and I finally grabbed a shovel to see what was going on. Turns out I had set my posts only about 18 inches deep when I built the fence back in 2022, and the whole corner was leaning from our clay soil getting wet and dry. A guy down the street who has built fences for 20 years told me he goes 3 feet down with a bag of quickrete per hole, and mine were basically sitting in mud. I spent last Saturday digging out two posts, resetting them, and rehanging the gate. Has anyone else had to redo their own work like this, and did you go deeper on every post or just the corners?
Sounds like you found the real problem which is that clay soil. Going deeper helps but the bigger fix is widening the hole at the bottom so it bells out, that way the concrete has something to grab onto when the ground swells and shrinks. I redid all my posts after one corner went bad, no sense waiting for the rest to follow.