Jan 16, 2019, 19:1201/16/19
Sometimes a malfunction occurs in the game, and this causes a problem.
Normally, you should be able to see a number of icons at the top of the screen above your hero and influence, the last one is of four arrows going out to the corners, this is what you need to be able to see to go to full screen mode.
Other than that you probably need to adjust your screen size.