Hi Ludwig,
Thank you for your reply.
I did the following configurations:
1. Defined the system email templates to be used for the collection process automation
2. Set up the collection process in Credit and Collections module > the process hierarchy, process details and the relevant business document email templates


3. Created a collection process automation

During testing, process automation was completed and I was able to receive the emails for this process automation, however, the results were as below:

Some had actual content based on template while others were just blank emails - no subject and no body, even though some share the same business document template.
I am not sure if I missed a set up but the other seem to work.
Thank you for your help.