Mine has never moved, but then I use a Tru-Turn spinner, and they have really good knurling. If you use a spinner that the knurling isn't as good, you can take a couple of disks of coarse sandpaper and glue them back to back, and that will solve the problem as well. Make sure you use a good wrench and tighten the bolt well. If you're using a wooden prop, you're going to have to keep checking it as the wood will compress.

HTH
Jim