Hi Joel,
I've encountered this exact issue. I have two mounts, both with SF 6PX Tactical lights. Upon breaking in my M4's, I noticed that both of the lights quickly worked their way forward and necessitated me removing them so they didn't fall off. I was thinking that I could use some grippy tape to secure them, by wrapping the rear of the light a little and then re-securing it. But if you have another option I'd be interested.
Also.... I was thinking about how I'd like to have the left side (the one with the qd sling mount) to be more like the right, as an option. This would allow the mounting of an X series weaponlight or something similar on the left side. It would also potentially allow the sling to be mounted 90 degrees from the barrel as a result of having a little more room to play with, which would be more comfortable for some people. Perhaps as an optional left side assembly sold separately? Just a thought.
Many thanks,
P.