Screen dims when on power chord plugged in

[Cody8]

My screen dims when I plug in the power chord. I've checked all the settings for the brightness when on battery and when not on battery and I've also tried using the fn+up/down to change the brightness. When I use fn+up/down, it shows the brightness SHOULD be changing but it is not and as soon as I wait a minute and try again, the bar is right back down to zero brightness. ANY SOLUTIONS??? Thank you very much for any help