Is it sliding off loose, or just moving a bit?
If it slides up and down, use some glue to lock it down. Slide the collar part way off, spread some glue on the leather near the top, move the collar down part way, wipe off any excess, push it the rest of the way down, clean up and excess again and let cure.
If it is just wiggling a bit, you may be able to get a few drops of cyanoacrylic to wick up and make it snug.