Can't clear my own signature

I have a document that I update every month and digitally sign with a certificate. Usually I start by deleting my signature from last month, edit the document with the updated information, and sign for the new month. This month, I am unable to clear my previous signature. When I right click the certificate signature box, it does not give me the "clear signtature" option and on the "verify signatures" tab on the left, the "clear signatures" option is greyed out.