This is a weird thing.
I installed adobe flash player successfully on my computer and I can watch YouTube videos. Yet, every time when I turn off my computer, and use it again the next day or whenever I want to utilize it, I was asked to install the flash player again! It seems as if it has never been installed!
Fortunately, I got one workaround for it; at least it works for me.
Try to turn off User Account Control on Windows 7 and restart the computer. There’re detailed instructions about how to do this in below pages.
So you can give a try for this possible solution when you encounter such a problem.
Reference bug link: https://bugbase.adobe.com/index.cfm?event=bug&id=2962851
Ru Wan, QE Dev Intern in Flash Runtime Customer Experience China Team