Try using Quick Set and Intel Graphics (if you have it)to change it if the Function key plus the Up Arrow key won't do it. Intel Graphics has Power Settings button that might help. Uncheck both power saving options there.
For some reason, I am unable to raise the brightness to max when my 1720 laptop is plugged in (and the battery is fully charged). I am, however, able to raise the brightness to max while on battery (or after powering up and plugged in and charging). If I raise it to max while on battery and then plug it in, the brightness goes down automatically and I am unable to raise it back to max, unless I unplug it again.
I've checked my power plan/brightness settings and cannot figure out what's wrong. Thanks ahead of time for your help.