Unable to Annotate Document

Even though I have confirmed I have annotate permissions in the file cabinet, I am unable to annotate some of my documents. 

All of the documents have additional docs clipped to them, which I can annotate, however the main one in my clip cannot be annotated.

Can anyone offer any suggestions on how to resolve this?

Thanks in advance,

Ross,
the most probable cause would be that these documents have a stamp applied to them that was configured as a 'signature' stamp - that would preclude any further annotations