This has happened to me as well, I put it down to my error. I think that sometimes I do not let up enough for the dog to catch in the shift mechanism. I have felt this happen when up-shifting and it is almost always when I don't let my toe relax enough. Have you tried pre-loading the shifter with your foot, to see if that helps?
'12 Vegas, 78 Bonneville, 69 BSA project.