I had this problem when I first tried to unlock my Duron Morgan, the HB pencil I used was'nt soft enough so I tried different grades of graphite : with a 2B, the multiplier stoped working after a few reboots, 4B and 6B worked perfectly well, I did it in april 2002 and the multipliers are still unlocked up to this day.
A word of caution, 4B and 6B graphite is very soft so handle it with care or you'll end up with a messy job. Sharpen the lead with a sharp edged tool like an x-acto blade and clean it with cloth to remove the dust. Pass 3 or 4 times on each bridge without pushing too hard. once you're done, hold the CPU upside down and gently tap on it to remove any residues, DO NOT BLOW ON THE BRIDGE or the particles may end up connecting the bridges together.
Fok Speling Misstake