I cannot answer this since you didn't provide logs or system specification so anything said would be just speculation. You find log files in your MediaPortal\log directory.
I have attached the log file. I have also attached two screenshots to demonstrate the problem. I am clicking on the window on the right hand side in order to maximize it ("untitled.jpg"). Not only it doesnt maximize but it also stays on (over the controls) as in "untitled2.jpg"