Any ideas on how to keep concrete garden borders in place?

That may always be a problem with the weight of the concrete pieces on the soil. Each time the soil is wet it will allow the weight to settle a fraction more. The sand underneath will also sink into the soil.

Here, we've used rebar pounded in as a method of keeping them from shifting. If they're moving outward, put rebar in front of them.

you might try using pavers underneath them, laid out flat. Picture an upside down T. The pavers' additional width will help distribute the weight. May slow down the sinking somewhat.

it may have taken yrs but we don't mess with ours at all and they stay even now Maybe u messing with them all the time is making it continue.

I would try pavers underneath