Sounds like then the circuitry that controls the write laser may not be working right. After you burned the DVD did you test it to see if it worked? I don't know what you burned to the DVD so hard to say what kind of testing you need to do.
If this is the case you can try contacting Plextor to see if they can repair them. I doubt they will and I don't know of anyone else that works on the drives. Like everything else in electronic components they are essentially all throw aways.
If you want to replace them I would suggest MTU's drives as they test every one before they ship them to make sure they will read CDGs. NEC/Sony Optiarc are the drives they use now since Plextor stopped supporting CDG (at least they have stopped listing it as a supported format).