Dymo Labelwriter Twin 400 print problem

We use this printer mainly for printing stamps with Endicia. Using MacOS with Leopard, and have the latest Dymo driver loaded. It's set tothe correct label (left side).The roll is installed correctly, bluelight is on, but stamps consistently do not print out the correctnumber of stamps. If we request four stamps to be printed, one or twowill print, then the others advance and are blank. We have uninstalledthe software, reinstalled, unplugged, cleaned print heads, and I'm at aloss. Our other Dymo 400 works perfectly. The unit is out of warranty.
I hope someone has an idea.