Automatic brightness

keeps unchecking its self any reason why

do you check it , leave the screen come back and its unchecked, does it happen after an event like plugging in a charger or mounting SD card or after reboot?
do you have any power or brightness widgets or apps installed?
are you using a stock or cusom rom?
 
This may be unrelated but have you read the post about how when you hold your finger on the notification bar, let it start to come down, then swipe your finger to the left (dimmer) or right (brighter) to adjust the settings?...I'm not 100% certain but I think if you adjust it that way, it overrides the auto brightness setting.