Display doesn't always turn back on
I have a Mac Mini connected to a Lenovo ThinkVision L220X LCD display using a Mini DisplayPort to DVI adapter.
I also have a MacBook Pro connect to the the same display using a Mini Displayport to VGA adapter.
I can switch the display between the two Macs using the switch on the front of the display.
I have the Mac Mini set up to turn the display off after 10 minutes and I can also turn off the display by moving the mouse to the lower right corner. Most of the time I can turn the display back on by hitting any key on the keyboard but every now and then the display will not turn back on no matter what I do. The Mac Mini is still running and I can use Screen Sharing on the MacBook Pro to get in and restart the Mac Mini.
How can I stop it from not allowing the display to turn on?
Mac mini (Late 2014), OS X Yosemite (10.10.5)