well.. i guess you are doing something wrong *LOL* - does your legato play work on other strings/frets? or is it only that specific pulloff? does the 12th fret sound at all - i mean when you normally pick it? if not, then there might be a problem with the frets or the fretboard... but i guess you simply mute the string unintentionally when you pull off. imagine a pulloff as if you where plucking the string with the finger that you pull off - as if you where plucking the string with your right hand. incase you fret the 12th fret correctly and you do not mute the string, it should sound right.

did i make myself clear? i guess no *LOL*