Messages

Hi,
I've been using the p89v51RD2FN...A0 for some time now, and always managed to program it using flash-magic.
Recently I got several chips ending with AC, which I can't program.
When replacing the chip in the socket to an A0 ending chip, I can program it and work with it fine.
The full chip numbering I have is:
Non working chip:
P89V51RD2FN
FA68-0523
-zG1420 AC
Working chip:
P89V51RD2FN
FA68-0459
-zG0731 A0

Do you guys have a clue?
Is it a malfunction of the chip-series? Or something I am doing wrong?

Thanks