Ninpopo, I am a very stiff person. When starting I couldn't kick above the knee; seriously. Then in time I was waist high and somewhat to the chest. Then easily to the chest but very rarely to the head if at all. After only doing 20 minutes, 3 days a week, first thing in the morning upon waking, I was able to kick many times to the head, but not all of the time, and it took some work. It mostly worked when I least expected it or tried to be honest.
But here is the thing and you've heard it before; you don't need to kick to the head. I relate kicking to the head with shooting a gun. When qualifying we have to hit the head but the body is more important. Why? Because it is the biggest thing and you have more chances of hitting it. Whether shotgun or sidearm the body is the biggest target and you have more chance of hitting that then the head; the same with kicking. And in a real altercation I would NEVER consider kicking somebody in the head and probably wouldn't even hit to the body as probably a little fearful of it being caught if they are aware. Instead I'm going to kick to their legs; their knee specifically. [I hope I didn't jinx myself as I have to work tonight and Saturday night]
Sure I would have loved when training martial arts to have easily kicked to the head, but that wasn't happening. But the head is over-rated and such a small target. Plus your base is compromised during/after the kick which could work against you. And in a real life encounter, you may wind up on your a$$. I'm not saying don't try to work for this but do it smartly; and a machine is not a smart way.
"IF I COME ... I'M BRINGING THE PAIN WITH ME"