Sounds like you just need to practice, try this on any string, fret the 8th fret with your index, 10th fret w/middle, 12 fret with the pinky, pick the 12th,po10,po8, do that over and over, concentrating on making that 8th fret note as loud as the 10th fret note, when you can do that say on the 1st string, start moving it across the strings, you could make a whole set of exercises out of this, like this,

--12 po-10-po-8s7s8-h10-h12-po-10--po--8--xxxx-----------
or

-12-p8-h10-h12-p10-p8-h12-p8-h10-h12--p8--h13-p10-h12-h13-

etc, etc just keep working on it and sooner than you think it'll be automatic..........