Signing with digital certificate - nothing happens

I'm trying to digitally sign one of our organizations forms in Acrobat Reader DC - but nothing happens when I click the box.  I last did this (and created a certificate in Aug 2016).   My supervisor is able to digital sign the form (space below where I sign) in Acrobat Reader DC, so seems like the form is fine.

I went into a previous (Aug 2016) form that I digitally signed, tried to use as a template for the new submission, when I go to the box where the previous signature is from last Aug, and click on it I get an error

Signature validity is UNKNOWN

The revision of the document that was covered by this signature has not been altered, however there have been subsequent changes in the document

The signer's identity is unknown because it has not been included in your list of trusted certificates and none of the parent certificates are trusted certificates.

Click Signature properties and then click View Sign Version to see what is covered by this signature.

I tried to make this a trusted certificate from there, but still doesn't work.