That is an old one. Look here:
http://www.subsim.com/radioroom/show...23&postcount=7
or
http://www.subsim.com/radioroom/show...15&postcount=7
It has nothing to do with the number of youre recruited gunners.
Only that you use the guns before. And if you do this, you have to leave them with the special Key-procedure. Yes, this is a Game-Bug.
Or do not use the Flak-guns yourself and let the Gunners make its job.
Greetings.