I'm new to this machine and still learning, but I'll try to help. I assume you are referring to the reel strip backlight. When they are not working, is it one particular reel or all reels? Opening and closing the door causes a reset which can clear errors. The fact you sometimes get the function back tells me its probably controller related, but I would not rule out a loose connection. It's the cabinet controller board that controls the reel lights. The board at the top back wall.
You can conduct a test of the reels and reel lights using the dip switches on the cabinet controller board. See the attached document for instructions (Normal dip switch positions are all off). You might also explore the machines diagnostic menu for any recorded errors or anything in the menu that might help.