If a trainee has accepted their emailed invitation, but they haven't started the training, chances are that they got busy and forgot about it. We send them a reminder after 7 days, and we send you a notification that they haven't started after 14 days (unless you've turned it off).
Beyond this, you can send them a reminder manually.
Go to Manage > Trainees and courses, then click on the number in the Enrolled trainees column to view the trainees. Find your trainee and click on Options.
Note that you can't do this for trainees who don't have an email address. You will have to tell them in person.