Allright, I've made some tests.
It looks like Lemon was right - the problem is indeed mostly in Flak HQ.
I've tested it on map that is 1v1 version of Lanzerath Ambush (I don't remember name).
I've noticed strange behavior of AA guns in general.
Sometimes they let plane pass 1/3 of map and only then start firing. No surprise that in this case plane have high chances to reach destination.
But still most of time they start to fire as soon as plane arrives.
No Flak HQ:
7 attempts
got fuel 5 times - 2 times AA started firing with delay (maybe more, because at first I didn't noticed that)
plane crashed 2 times
With Flak HQ positioned in sector next to base sector:
8 attempts
got fuel 2 times - 1 time AA started firing with delay
6 plane crashes
The speed of the Sherman is its advantage. However the fuel drop ability needs some fixing. Coming over a random direction would be better.
That could be a solution, but this game already have too much random things
