This post refers to 5 HUD Pack
there are many issues about HUD not working like this 
the cause is simple because the variable section is empty like this 
how to handle it? The trick is to enter a variable part to your plane like this 
How to enter Variable code into your plane using xml file?
android/data/com.jundroo.SimplePlanes/files/your aircraft
then go into xml and enter the variable, the way is at the beginning you want to put it you have to do like this <Variables>, then to end it do the same thing but a bit different like this </Variables>.
Example:
The Variable Code
HUD variables to edit in the XML file
Another Option
If you want it even more difficult then you can copy and paste from within the game.
Variable Code
HUD variables to edit in the game
and that's all I can say, so if something isn't clear, you can ask the moderators.
@Geometry Dash World Very helpful. Thanks for sharing this helpful post.
Thanks, helped a lot
@ollielebananiaCFSP sometimes it often happens in several conditions
I mean, all these guys who made one part HUDs wrote it in their descriptions that it's necessary to copy the variables, people are just lazy and don't read anything. That's often the case when it doesn't work, just lazyness.