Just stating up front that I am a new member and not an expert on this, but as no one else has replied and I've experienced this before I thought I would share my thoughts and suggestion
I've found when I had this problem it's because of the way my screen resolution was set. From what you describe, maybe you set the game resolution higher than your desktop so it's unable to maximize?
I would suggest that you ask for help on the Matrix forums as they will know exactly what might be causing this. Hopefully you've already gotten help