You may want to go to your settings menu and see if maybe you have any power saving modes turned on or if you have an auto adjust feature for screen brightness which may be turned on. Some power saving modes may dim the display to save power or your tablet may also have the auto adjust feature which can sense how bright of environment the tablet is in and will adjust the display brightness to a level more comfortable for viewing. Like in a low light area the screen will dim or in a bright area the screen will get brighter. So look through your settings menu and look to see if you have any of these settings enabled. If so try adjusting them. It sounds more like the auto adjust brightness feature to me.
If changing the settings does not fix the issue then reply back and I would be glad to help you troubleshoot further.