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#1
Clint is correct sir. You have to have the correct screen resolution set or it will be like watching a wide screen movie.
Right click on the home screen - display settings -resolution... Usually choose the highest one or the one that is recommended and you'll be good to go.
Do you mean just that app/program that you have open, or anything that can be maximized? I've had certain apps that even maximized did what you have happen. It just wouldn't maximize to full screen. It must have a limit built in to keep a certain aspect ratio. If you mean all windows won't maximize I would agree with the previous posters, and say change your resolution/scaling to see if it affects it.