well, i fly the p38J almost exclusively.......so i may need to "read" the other guy a little better, or just react quicker.....i know other guys in 38's do it well.......
It's a refinement to your technique that's missing, that's all.
Getting the guy to follow you up, and stall out under you, is just a part of the process. As you're seeing, it doesn't actually accomplish anything beyond that. It doesn't give you the kill...
"Reading the other guy" is very important. It's part of the ongoing, all-encompasing SA required to succeed regularly. Being able to accurately read his E-state, capabilities, etc, and weigh them against your current possibilities is a basic requirement. You can't rope a guy who's going to catch you on the zoom, regardless of whether he's faster, or can cut across the "corner" you create by going too vertical too quickly (even if he's slower than you).
The first requirement might be to assess whether you CAN rope him. Then whether you SHOULD rope him (other bad guys around?). And then it's still up to him whether he'll LET you rope him.
If all that goes ok, and he's roped, you still need to be set up properly to score the kill, and that's actually the most difficult part. You want him to stall, but YOU SHOULDN'T. You want enough speed to maneuver over the top. If you stall and flop, time is wasted before you can recover the ability to line up your shot. By that time, it may be too late. I like to be coming over the top with about 130mph, generally assisted by a notch or two of flaps, which I retract AS SOON as my nose is down, because they screw up my ability to line up the shot.
Position is important. He can't be too far away on the horizontal plane, but I don't particularly like him directly under me either. The "final placement" of his stall in relation to you is important (and if you're not stalled, you CAN adjust it by adjusting your own position).
Stalling under you isn't enough either. HOW FAR under you matters. Stalling 600 below you is too close, and he may even pop you from there. Stalling 2k below you is bad too, he'll be nose-down and maneuvering before you can reverse and shoot him.
What I like, is a guy who's on the verge of stalling at about 1200 to 1.5K below me, while I'm still going up at about 130. There's a lot of "judging" going on, because I want to come over the top just as he hits his stall, so as he's just started to hang, my nose is already pointed back at him. This means beginning my reversal BEFORE he stalls, but not too soon, or I may get shot. Timing is crucial, as is the ability to judge his situation. With my nose down, I want to see him hanging about 800-1000 below me. That gives me just enough time to line up a shot and kill him. I also need to be under sufficient control to dodge his debris, or him, if I've mis-judged. He should be hanging/flopping and helpless when I take my shot.