A slab that rocks is usually the bed
I stepped off the shed on a February thaw and the doorstep slab clicked. One corner went down. The opposite corner came up. I thought the slab had cracked. I lifted it. The slab was fine. Under it was a hollow in the sand, a wet pocket the frost had opened. I had laid that slab on a thin, uneven bed the autumn before and tapped it until it looked flat. Flat is not the same as supported.
A slab is a lid. The bed is what it sits on. On a small UK plot that is usually a layer of sharp sand, or a lean mortar, over something firm. The bed has to touch the whole underside. A hollow is a hinge. Your weight goes on one edge, the slab rocks, water runs in, frost opens the hollow further. People blame the slab. The slab was doing a lid's job on a gap.
Gravel that sinks is usually the base is the path beside this. Loose stone wants a compacted layer under it. A slab wants that too, and then a bed that does not leave a void. I do not pour a bag of sand on clay and call it laid. I check the ground first. A fork and a spade are not the same digging tool. The spade cuts the area and gets the soft muck out. The fork loosens a packed patch without smearing wet clay into a pan. If the soil under the sand is a sponge, the bed will follow it down. Then every slab rocks in March.
When I check a rocking slab I stand on each corner, then the middle. A click is a hollow. I lift the slab and look. Dark wet sand, a crater, or a clean patch where nothing has touched the underside, that is the fault. I refill, compact, and set the slab again so it sits, not so it looks level from a step away. A long straight edge across two slabs tells you more than your eye. If one corner is high, I take a little bed out. If a corner is low, I add, then tap. I do not smash the slab to make it obey.
Path edging that lifts is usually the frost is the line around a paved bit. Frost levers a shallow edge and it also opens a hollow under a slab. I reset the bed, not only the edge. An edge that has kicked out lets the bedding wash. Then you have a rocking lid and a spilling line, two jobs, one wet week.
A raised bed that bows is usually the rails is the box you must not use as a paving edge. I would not run slabs up to a thin rail and let the bed spill under it. The rail is holding soil. It is not a kerb.
Skip five blobs of mortar and a slab sat on top. The blobs crush or drop, and you have built five hollows on purpose. Skip laying on bare soil because the slab is heavy and "will settle". It will settle into a rock. Skip a bed of builder's sand so fine it pumps to mud. Sharp sand holds a shape. Skip tapping only the middle until the corners rise. You have made a hill. Skip leaving a rocker because it is only one slab by the shed. That is the one you step on in the wet, with a tool in each hand.
This shows after frost, usually a February or March thaw, when the hollow has water in it and the click starts. Autumn is when I would lift and reset, while the ground is still workable and before the wet months sit in the gap. I walk the paved bit after the first hard freeze. If a slab moves under my heel, I mark it and come back in a thaw. I do not smash it down on frozen ground.
I do not need new slabs because one rocked. I need a bed that touches the whole underside, on ground that will not swallow it.
If I later add a buy link here, it will be a paid promo.