Just realised my stuka gunner hasn’t fired a shot in a couple of days… I know someone else had an issue like this 3 pr 4 weks ago (?) with Breda 99 gunner…
Same here.
This has been reported and known for several months already.
I myself did report it on CBR.
My separated tank machine gunners have never fired a shot on their own either. 
That is not a bug but a feature.
seems to fall under “Teaching the AI how to handle heavy weapons”.