I would look inside that tailcap because the small tabs on the contact ring break off. It may not allow contact to be made when screwed down without the extra force from pushing the tailcap further.
There are 3 small chrome tabs that look like fingers. If you've had it for a long time, they maybe broken. Surefire can warranty the tailcap for you. I have several L1's and A2's that have 1 of the tabs broken off, but still function because there are two still functioning.
Look at this photo. The small finger-like tab that you see here is complete and unbroken. It usually breaks right at the sharp bend.