Are you still using Acrobat 8? If not read further.
All Acrobat versions prior to 11.0.07 had a security flaw that they allows signing with certificates that had "Extended Key Usage" (EKU) restricted to certain certificate uses and those did not include document signing. Most frequently those were certificates with "Client Auth" or "Server Auth" in EKU. Acrobat/Reader 11.0.07 fixed this problem, which also means that while previous versions accepted such certificates for signing 11.0.07 and later do not.
If you are still using Acrobat 8, your certificates may have expired.