Hi Rachel

This may be due to the set up of the Certification - the Assignment due date may be based on an event such as a number of days after first login so not able to be changed.

The actual due date for assigned users can also be extended only 

Do you think this explains why the dates aren't able to be changed - to investigate further are you able to create a support ticket with details of the certification settings and version of Totara. 

Regards