The problem is that when the form is minimised, it doesn't have the focus - so it doesn;t get any keyboard activity.
If you want to catch keystrokes when minimized - and I'd advise against it, it annoys users if the keys are used by other apps like F3, F4, and F5 often are - then you have to start doing very complex code to install a Global Hook:
Google[
^] - but do not expect it to be easy, and do expect your attempts to crash your PC or leave it in a very unstable state a couple of times while you are developing it.