Dvd recorder can't read disc it previously read

I have a panasonic dvd recorder Model# DMR-ES40V. I have been copying programs off of my dvr onto DVD-R discs with my dvd recorder.I have put things repeatedly on the disc and played them back to make sure they recorded before deleting the progrrams off the dvr. I put the dvd in to add more shows and now the recorder says it cannot read the disc and disc is noncompatible (it doesn't appear to be scratched or damaged). I have tried re-setting the recorder defalult settings, unplugging and re-plugging the recorder,and cleaning the disc with soft cloth. I don't know what else to try. Other dvd's still play in the recorder , just not this one. I would love to get this dvd finalized so I can play this on other players. I do not want to lose what is on this dvd(shows and performances that I will never be able to get again) but can't get the recorder to accept and read it so I can finalize it. Any help will be greatly appreciated.