A fence post that leans is usually the ground
I looked at the back fence in March after a wet winter. One panel had a tilt you could see from the path. I thought the post had snapped. I put a hand on it and the timber above the soil was sound. The hole was not. I scraped the earth back and found a wet crater. The pack had washed to soup, and the post was standing in a cup of mud. A March wind finished the lean.
A post is a stick. The hole is what makes it a post. On a small UK plot that hole has to be deep enough, and packed tight enough, that a winter of wet soil cannot turn it into a hinge. Earth, hardcore, or concrete, the job is the same. Fill the void so the post cannot rock. If the pack goes, or was never there, the first wet month opens a gap. Water sits. The next blow moves the post a few millimetres. Then a centimetre. Then the panel follows. People blame the timber. The timber was doing a post's job in a hole that had given up.
When I check a lean I start at the soil line, not at the cap. I scrape back and press a spike into the timber at ground level. If it sinks, the post is rotting there. Gravel boards rot at the soil line is that wet on the sacrificial board. A post can go the same way. If the spike only marks, I still wiggle the post. Movement at the base with a sound stick means the pack has a void. I look for a crater, loose stones, or a concrete collar that has cracked and is now a bowl. A bowl holds water against the post. That is a rot trap and a lean.
Depth matters. A hole you can dig with a trowel is a plant hole. I want the post in far enough that a boot on the panel cannot lever it out. I will not invent a number for every soil. Clay that is soup in February needs more depth and a pack that drains, not a shallow plug. Path edging that lifts is usually the frost is a shallow line the frost levers. A post is the same physics on a taller stick. Frost and wet both work the hole.
A raised bed that bows is usually the rails is the box you must not brace a leaning fence with. I would not screw a rail to a post to hide a lean. You load the bed and you still have a hole that moves. A compost bin is a box, not a bag of compost. A bay tight against a post keeps that soil line wet on purpose. You get a rodent hotel and a lean you will find in March.
Skip a longer post in the same wet crater. You have planted a taller hinge. Skip pouring mixed concrete into a hole full of water. You have cast a lake around the stick. Bail it or wait. Skip a metal spike in winter clay that will not take a drive, then calling the lean the spike's fault. The ground would not take it. Skip packing with the spoil you just dug if that spoil is smearing clay. It will polish and then wash. Skip leaving the panel on a post you already know moves. Take the panel off before the next blow turns a lean into a fall.
This shows after a wet winter, usually March, when you first look at the plot properly. Autumn is when I would reset a pack, or dig a proper hole, while I can still work the ground. I walk the fence after a long wet week, not on a date from a catalogue. If a post moves under my hand, it will move more by February.
I do not need a new fence because one post leaned. I need a hole that stays packed when the soil is wet.
If I later add a buy link here, it will be a paid promo.