Excel 2003/2007

I use a VeriSign digital signature in my VBA project with an
expiration date of 2012, and I've used it since Jan 2009. Suddenly
when I saved I got, "There was a problem with the digital certificate.
the vba project could not be signed. The signature will be discarded."
Resigning took the certificate but repeated the message when I saved.
I reinstalled the cert from the original pfx file from VeriSign and it
seems to be working now.

Can anyone shed light on why this might have happened and what I can I
do to prevent it? It was scary because I distribute this add-in and
it'd be a huge problem if the cert failed on me.

Thanks,
Mel