Hi, Just in case if you have not tried , try this.
Go to control panel and programs and features, if you click, you will find turn windows features off and on.
Click it wait for it to load, and then go to item media , expand, and then find windows media player there, just uncheck , reboot. Then on reboot, go to the same menu, enable it allow it to change the settings and reboot. Next time, you try to get full screen by double click any video.
This is one of the trouble shooting of wmp