Hard to say. It was on my old low rider. Happened most often when my I was riding the tank until it was lower.

 

I cut out a nice clean round piece of 3m carbon fiber vinyl and covered the OE cap. now it looks like a $100 cap. woohoo. Nah seriously though- I have used the flush caps from HD and e-bay. they always get stuck. at least for me. I've never had the stock cap get stuck but I don't over tighten it either. just snug is all.

 

I've really wanted the flush caps for mine, but like everyone else, I'm afraid they'll get stuck. My stuck cap rattles at certain RPMs and is very annoying.