Dual layer problem

HI,

A couple of months ago I burnt 3 different media types on Aone DVD-Dual layer media @ 2.4x using a Pioneer DVR-111L under Nero & Magic Iso.
1. Backup of game
2. Backup of mp3/wav/ogg
3. Backup of musical dvd
All 3 backups passed data verification.

All backups succeeded though a problem has occurred, at this point backups 2 & 3 seem to be unreadable in layer 2.
How do i know? Well i watched the files being burnt and at the beginning of burning layer 2.

What I can't understand is that when i first burnt my data it was fully readable and working perfectly now at some point these backups are useless only layer 1 is readable. All discs are well kept hence no scratches at all. Also until now backup 1 is still ok dunno next, probably the media i'm using should be changed.

Dvd-r/+r/-rw single 4.7Gb no issues at all