sorry, i dont open or download any zip files.
But have you maybe tried play with physics options, like setting proper gravity?
bulletAppState.getPhysicsSpace().setGravity(new Vector3f(0, -20f, 0));
or:
bulletAppState.getPhysicsSpace().setAccuracy(0.01f);
?
there are a lot of settings, also for collision shapes too.
Also this looks like you use this framework:
so i assume you already talk with @remy_vd
and im also not sure why your file is under “rvandoosselaer”, do you cooperate in this project or what?