Unfortunately the RPC-1 site appears to be down and we don't know when it'll be up again.
It's possible that your drive needs a clean since something may have stuck on the DVD lens. Otherwise it does happen that the DVD optical pickup unit (OPU) has failed, and only CD read/write works because it has a different OPU.
To cure the former possibility, gently brush the lens with a clean paintbrush. For the second possbility; if it won't recognise commercial pressed DVD movie discs, unfortunately there's no cure except a new drive
(@Toking14: the LiteOn SOHW 1633s is a computer DVD writer.)