what did you use to rip the files in the first place? or is it possible they have DRM crap on them?
i'd say re-rip the original CD using a program designed for ripping like EAC.
what type of dvd did you use? it's possible that the burn was of such poor quality that the material on the disc is playable but not retrievable in order to rip. this is a very common symptom of poor quality media. please tell us the media code of the blank discs you're using. media code can be found by looking on the disc info tab of nero cd-dvd speed.
also what burner model and firmware did you burn with?
what was the rated speed of the media?
what speed did you burn at?
we need more info!