Since this only works while you jump, it seems to be an issue with the character wanting to stay on the ground. You could possibly try setting Humanoid.PlatformStand to true while they are being moved, which essentially makes it so the character doesn’t move on it’s own. This allows the rocket propulsion to take control and move the character where it belongs.
If this doesn’t fix the issue, you could always just force the user to jump by setting Humanoid.Jump to true.
Please let me know which one of these (if any) solves your problem!