Mid belts, lower belts and even advanced belt all do one thing wrong when it comes to distance, they step before delivering the technique. Most times they are in perfect range to deliver the perfect technique but for some reason they take that one front foot step that now puts their distance to close. Just watch them and you will see it just about every time.
Even when kicking focus pads for targets they all take a front foot step so it becomes habit. You must break this habit first. When kicking paddles or shields or anything that is not hitting back make sure they don't step. Get your proper distance and simply kick.
I think that many do this to get some momentum for the kick. If this is the case then they don't know how to initiate the kick in the first place. They use the step to build the power. The power should be from the push off, not a step.
Let me know what you see when you have your students kick a stationary object. I bet they all step. Not only does this throw your distance off but it also slows down the kick and telegraphs it as well. So when they step the opponent either slides in or angles off and counters with ease.