Well--probably nothing wrong but I'm not willing to join your crew just yet

.
From what you said I think you might be trying to come in from DIRECTLY AHEAD of the convoy. The closest I want to come to that setup is to do a head on approach on a flank merchant of a wide convoy (say 6 columns wide). Even then I worry. A lot. Try approaching more from the quarter--that is come in from the side of the convoy more than from in front. I tend to worry more about the flanking escorts than the leader because of the way I come in. And sometimes I get lucky and the closest flanker is out a few thousand yards looking at kelp

. That opens up the whole flank for me to pick on and gives me time to set up my shot.
The downside of this type of approach is it's easy to misjudge ranges and it can leave you out in left field. You have to try to time it so you come in kind of behind and to the side of the leading escort--but far enough ahead of the flanker so you can turn down the columns of merchants. It's a method that has served me well since Aces of the Deep.
If you'd like me to try to clarify my points, ask away. I'll try to help you the best I can.
Hang in there! These sims take Practice with a Capital P!!!! But the pay-off is that smug look when you finally go to bed LOL.
Cheers!
Peto