i know some drives have problems reading certain Blank media (memorex,sony to give a few examples) and also some drives after certain amount of burn, stops reading disk for no apparent reason.
can be that probably your drive is going defective, is it new ?
and about blank disks if you don't upgrade your firmware you won't be able to read newer disks. for instance if you find you're able to read lower speed rated disks and not higher speed rated disks then it's a firmware issue