The problem I most consistently see with front tucks is that the gymnast looks down. This makes the flip go down and often makes it stay in one place. You don't want a front tuck to travel forward much, but when done properly, they do travel some. As you are running and jumping for the punch, spot something out in front of you that is higher than you. This will help you keep your head (and chest) up. I assume your coaches have had you do punches up onto the mat. If not, you need to do a bunch of those. Run down, punch and land on top of a mat with straight legs. The mat needs to be at least 18" higher than where you are punching from. Throwing the block that gymfan mentioned works really well with this drill.