Learning Assignments - Mass Actions - Mark Activities Complete

Content

Within Learning Catalog > Learning Assignments, options available to return 'Assignment Status' = All, and then choose Mass Action = Mark Activities Complete.
This however does not always mark all Names with a status of Complete, and some remain with Status of Not Started.
Have then also seen example of status changing from Not Started to Complete a few hours later, as though there has been a delay to the Mass Action taken. 
Are there are any documents or guides available that provide a comprehensive look at the updating of Learning Assignments? Are there specific Scheduled Processes that push along Assignment Status?