If you hit the jet pack at the same time as you hit the ground it will not fly you back up. You will see the same effects as you where flying.
This bug has been reported multiple times.
I didn’t find a thread regarding this issue, but another jetpack problem.
Thanks for your report! This has been forwarded straight to our Trello board. You can track its status there (it takes a couple minutes to show up on Trello).