If you have assigned an Author as a Map Admin, but the author is still unable to access the map in question, you will need to ensure that the user also has access to the map and diagrams via the Diagram Access Rights menu.

Please see the below extract from the user guide.

"Map administrators have certain administrative privileges on maps. For example, map administrators can modify map properties, change access rights, add and remove authorizers, request authorization, cancel authorization, add and remove reviews, edit and delete recurring reviews, add and remove acknowledgers, request acknowledgement and manage subscriptions. Only process authors and administrators can be assigned as map administrators. 

Note: Assigning a user as a map administrator does not automatically provide them with access to a map."
 

Author Assigned as Map Admin Cannot Access Map in TIBCO Nimbus