This is odd. I've never run into a problem with retention is RC sheaths. Actually I thought that thanks to the retention screw that you can reposition that it would be impossible to lose grip. Just simply move it to a different hole nearer the handle, further up the sheath and presto. If you can, take a picture or two of how you have your sheath set up. Maybe we can see and point out what to do different.
As a side note, I made several kydex sheaths and have used them for a long time. The only time i ever ran into trouble with them is if i had forces acting in another direction or if the sheath got damaged. Other than that, the seem pretty stable to me forever. Pretty much any kydex, injection molded plastic, etc. should last for a REALLY long time without any problems.