As the game progresses I often find myself getting into a vehicle with no means of firing. I can drive just fine but no reticle shows up and I am unable to fire. This seems to occur when you play a single match for quite a while. I'd say at least an hour or hour and a half.
A way to alleviate it is to get in and out of the vehicle until the reticle shows up. However I've notices this issue seems to only get worse as time goes on while in a match.
On a side note I've also noticed that firing in a vehicle with low ammunition, like a mammoth tank or something, while you're reloading if you click at the right time before you're reloaded you will reload again once fully reloaded rather than firing.