Multiplayer game can be extremely and lethally different from single player game in this aspect. If your enemies have researched sub visibility, convoy raiding and other related techs way ahead you need to take mirror countermeasures. That is, go for sub detection asap and also other convoy protection techs but detection is the most important one.
People also use lone wolfs - 20 enemy subs, one per stack, upgraded with 3-4 years ahead techs can ruin the whole game for the UK or the US...and even Japan. So you need to have DDs upgraded with techs way ahead (at least 2 years ahead) and split them into maximum fleets. 2 DDs per fleet optimal IMO.
Remember, extremely modern and dispersed DDs are the best way of dealing with sub-focused enemy.
Also remember, CLs have much lower sub detection than DDs.
Naval bombers are also good but they have limited range. If they could fly everywhere I would use solely them, instead of DDs.
One more thing - if you situation gets really nasty (like losing 10 or more convoys daily) then you need to switch to manual convoy control and turn off all resource convoys + non-essential supply convoys until you have proper anti-sub hunter groups.