You can work on opening your hips and hamstrings though, because alot of people in muay thai have practical problems with the roundkick, impacting correctly with the shin. It usually has to do with the tight hamstrings or hip, the shin impacts flat on the target, especially for the bread and butter, mid line kick at the opponents rib, elbow target line. If you look at a photo or some video of someone doing the kick, you can see how flat the shin lands for maximum impact. In thailand, i know the trainers worked with many foreigners getting them to get their kicks impacting correctly. In time you work on the kicks and flexibility and you can go higher.
You can also use basic yoga dvds or classes, for hip openers, and various hamstring stretches you can work on over time to help with flexibility.