Maybe it's the way I'm reading the comments, but it sounds like it's not ankle flexibility or hamstring/quad/ankle strength that's the problem.
It sounds to me like it's either calf strength or an inability to tense the calf while the quadricep is tensing. These are on opposite sides of the legs and is a bit subconsciously counter-intuitive.
I would say concentrate on tightening your calf (to point the foot), lift your thigh and just practice extending your leg out at lower heights (knee/thigh level) and then pulling it back. Don't do it fast just stand on one leg and keep doing it - changing the angle of the kick between front kick and turning kick and everything in between. The key is you're training your body/mind that it's normal for you to keep your calf locked contracted/tense (so your foot is pointed) while your quadricep/hamstrings are responsible for straightening and bending your leg.
After a while (couple of weeks) raise the height to waist, then a couple of weeks chest, then a couple of weeks head. You'll find your leg muscle strength for high kicking improves this way and you should be able to maintain the tension to keep your foot position.
Doing this (keeping the foot pointed) is harder at high heights purely because so much more of your concentration goes in to getting the kick high that it's easy to forget to maintain that tension. Note - it doesn't have to be bodybuilder-shaking tension just enough to keep the foot pointed straight.
Good luck.