Ah. Air dribbling. That's one of the few things that, mechanically, I haven't quite gotten down yet. I always find that when riding the ball up the wall, I'll hit it off too fast, or hit it straight out instead of slightly upwards, or I'll feel as though I could've totally had it if I wasn't using the Mantis. The YouTuber's I watch are able to do it at such insane speeds that simply look impossible to me. Maybe it's their higher framerate? Because I could turn my deadzone all the way down, and there's no way in hell that I could air-roll as fast as what they're doing.
Imagine on the ground when you drive straight into a ball. Your car's vector and the ball's vector are drastically different. A collision under this condition results in chipping the ball up and taking an arc trajectory.
Turn that visual 90 degrees and that's essentially what you do on the wall. The key difference is that chipping the ball off the ground requires extra force to counter gravity directly, hence chipping off the wall doesn't require as much force to get it to leave the wall.
What you will want to know in regards to the problems you're having:
If the ball flies straight out, it's because you're not getting under it enough to pop it up. In other words, you're hitting it horizontally or downward.
If the ball flies too far away from you, it's because your speed doesn't match the ball. This is more than likely because you've hit it too hard. Either control your speed before you touch the ball, or boost a bit up the wall after hitting to give yourself sufficient vertical momentum to catch up the ball. You'll want to jump off after and use the rest of the jump's momentum to propel yourself towards the ball.
Personally it's a lot more intuitive if I go at the ball with a diagonal path rather than matching the ball's elevation while on the wall only then to jump and boost. The issue I find with the latter is that now you don't carry any upward momentum to approach the ball with to dribble. So I find most success if I hit the ball while it's still on the lower half of the wall, and wait a tad to give myself some room to fix my angle once I leave the wall to catch up to it. Just be sure to hit the ball after it has traveled past the curved bank for consistent results.
Also while I've never driven the Mantis, some cars do rotate faster than others.