Yes, the jetpack is tricky. First, make sure that you are out in the main castle grounds, well away from the castle and any pathways. Then click on the jetpack to open it.
Place your cursor to the left of, and just slightly below, the halfway mark on the lever control. Then click-hold, and drag your cursor up a little. The lever moves straight to halfway.
If you simply double-click at that point, although the lever moves up without stopping at the quarter-mark, the game doesn't like that method, and...kaboom.
You might also double check that all your jetpack instructions sheets are printed correctly.
"Bleat, Watson -- unmitigated bleat!" ~ Sherlock Holmes