Memorex 4x DVD-R weird compatibility

I've been using these discs with the NEC 1300A and Nero 6 for a while now and have had great results. I was so pleased that I ordered another 50 from a different supplier. However, not one of the new discs is recognised. Nero reports some kind of seek error. I still have some of the old ones and they are fine. I ran old and new through DVD Identifier and the results are identical. One thing I noticed is that the old ones came in cellophane-sealed cakeboxes, whereas the new ones were not sealed, had the Memorex insert missing in one box, and also contained 4 physically marked discs. Any ideas why these new ones won't work when they appear to be identical ?