Thank you for your suggestions.
Through the Amazon Kindle support desk, I have been able to resolve the problem.
The solution is to deregister your device(s) i.e. in my case my Apple Mac, and re-login into the Kindle app whilst allowing the re-sync to take place.
Steps are as follows:
- Go to Amazon.com and login into your account.
- From the 'Accounts and Lists' in the main menu, select 'Content and Devices'.
- Select the 'Devices' tab.
- Click on the Kindle icon to display the list of linked devices.
- Select the 'Deregister' button everywhere your device occurs.
Once the aforementioned steps have been completed, return to the Kindle app, login with your username and password and the service will be restored.
Regards
Troy