The corrections given here are correct. The issue I would have is that without seeing it, it is hard to diagnose the real issue. I have some guesses but don't want to assume. If you post a video here, or email it to me at
[email protected] I can give you a better answer and drills to specifically address it.
The way I/we coach it here is that the set for a tuck is no different than the set for the layout. Every skill and step leading up to it must be consistent. This does not mean doing it, but it looks and feels exactly the same. If a running tuck is not head height or higher when inverted (upside down in the middle of the air) then, we are not working layouts until that is fixed. Once it is fixed, we don't teach a new skill: we teach a new body position. The only difference between the tuck and the layout is the shape of the skill and not the skill itself. This is why consistency is the key. When you think and make it a different skill, you put extra into it - which often throws you a little bit off.
Another issue we don't like to address is if the ROBHS is not long, tight and strong enough it does affect the height in the tuck or layout. Simply punching the ground harder will not fix a lead up skill issue.
Absent of a video any thing that reinforces the correct layout shape will help you. Those posted above are all good ideas.