A few problems cause most authentication issues using certificates.
If a user cannot log on with their certificate, try one of these options to resolve the problem:
Verify that the user has not been disabled.
Verify that the certificate has not expired or been revoked.
Verify that the certificate is signed with the correct certificate authority.
Verify that the DN field is correct on the user configuration page.
Verify that the browser is providing the correct certificate.
Check the Audit Log for authentication messages.