When a fence starts to lean, most people look at the panels. The panels are almost never the problem. What is happening is underground, at the footing, and until that gets fixed the fence will keep moving.
Missouri gets a real freeze-thaw cycle. Water in the soil expands as it freezes, and if the bottom of a post sits above the frost line, that expansion lifts and tilts it a little every winter. A few seasons of that and the whole run is out of plumb. This is why post depth is not a detail — it is the entire job.
The second cause is a footing that never really was one. A post backfilled with dirt, or set in a shallow collar of concrete that does not extend below the frost line, has nothing holding it against wind load. A tall privacy fence catches wind like a sail, and every gust works the post loose in its hole.
The third is rot at the soil line. Wood posts rot exactly where wet soil meets air, not down at the bottom. A post can look fine above ground and be gone at the surface — if you can push a screwdriver into it there, the post is finished no matter how solid the rest looks.
The real repair is straightforward but it takes work: pull the post, dig the hole deeper than the frost line, set a new post in concrete, crown the concrete so water runs away from the wood instead of pooling against it, and let it cure before you hang anything back on it. Shortcutting any of those steps buys you a season, not a decade.
One more thing worth knowing: a sagging gate is usually the same story. The hinge post is carrying the whole weight of the gate on a lever, so if its footing is weak, the gate drops. Fixing the latch side does nothing.
If a fence in Saint Charles, St. Peters, Florissant, or anywhere in between has started to lean, call or text (636) 450-0017 and we will tell you honestly whether it is a repair or a replacement.