I have had this same problem.
I am a serial Re-Writer and have a stack of discs i keep shuffling through, none of them as of yet have failed on me.
Couple of weeks ago my main pc started telling me my discs were not RW... every single one, my other pc erases them fine.
I took it to my mates shop and he said that after months of abusing my DVD Rewriter using from everything to writing, erasing, listening to music, installing games and such, it had screwed my drive.
First get it checked, but my advice is get a new rewriter.
and don't chuck your old RW discs just yet.
**If all else fails you could just try the toothpaste trick**