There is a bug with the jetpack that I've experienced since beta, but only now have I figured out exactly what causes it. If, while you are holding the jetpack key (jump), your feet touch the ground or a solid object, your jetpack fuel will continue to deplete but will provide no thrust. Easiest way to reproduce this is to jump off something (that's not tall enough to kill you) and activate the jetpack just before you hit the ground so that it doesn't slow you enough to stop you from hitting the ground. This also happens to me a lot while climbing hills or structures when your feet just barely kiss the ground and can result in a deadly fall.
I get this a lot too. Try to sprint up a hill and then jetpack to fly over a portion you cant run up, use up all fuel and go no where :|
