Zenia M & everyone else; Yes, I found a solution. sort of...
I reinstalled and began a new career. When flak guns became available, the weather was almost always touted as the reason I, or the crew, couldn't use the gun. When weather permitted, it could be accessed, but the crewmember just stood there doing nothing, however I could jump in and fire at incoming aircraft. After upgrading from type IID to type VIIB, when in extremely calm weather, aircraft were spotted and I sent a crewman to the flak gun and, low and behold, he came through, firing at the incoming aircraft. However, this only occurred when the surface of the sea was calm with zero wind. I can't begin to fathom what's at work here; maybe it's fixed, maybe it only works sporadically. It'll be awhile before I can determine whether I or the crew can reliably man flak & deck guns.
Also, I'm going to have a look at weather mods that allow access to guns in any weather; I'm growing weary of denied access in any but the calmest of seas.
So, Zenia M, my suggestion is to reinstall & start over, get a flak gun and wait for calm seas and an attacking aircraft to see if the reinstall solved it; good luck..
