“Perform bulk action on selected people” within a course only allows you to select the 20 names on the existing page. If you highlight them and move to Page 2, all names on Page 1 become unhighlighted. You should be able to select everyone who has not completed the training and send them a bulk email
This query comes under the "Add page Numbers and/or "select all" function on the "people: pages".
Totally valid, not sure why there are no features to select at the top of a page, to do, in progress, completed uses and then assign training to a group or all. Once these filters are set we shouldn't have to keep selecting them after submitting something. Simple LMS function that is missing.