What they said... and one other thing that may help. When you are setting up your platform, take your pressure flaker and on both sides of your platform, take a small flake off on the opposite side you are going to be striking from. This will make your platform a little bit stronger and help channel the energy of the strike. As you are learning, it really doesn't hurt to make your platforms a little larger. It will give you a slightly bigger target to strike reducing miss hits. Also, abrade the top of the platform back just a little bit further so that if your strike is ever so slightly high, you still get a good hit. It may go a bit deeper than you intended, but with time and practice, it will start to be easier. And the truth is that even though I may sound like I know what I am doing, I am still practicing what I'm suggesting and still learning.
Look at the results of each hit. Put the flake back after it is removed and really check out how well you did in setting things up and if you got the results you wanted.
One other thing I might suggest. Looking at your pictures, that looks like it might be some pretty tough flint or chert. The harder the material, the harder it is for a novice to get the hang of taking good thinning flakes off. At least it was that way for me. I would suggest getting your hands on some flint that isn't quite as hard. It will make the learning curve a bit easier. There is a world of difference between the different types of stones.