We have had a lot of trouble with a client getting completion settings right - in activities and course settings, and confusion over certificate locking. At the start they didn't tick the checkbox for "completion starts on enrolment" and when this was ticked later, it took a while to realise that it is not backdated for existing enrolments and the only way to make previous users appear in reports was to un-enrol and re-enrol them.
We thought this was fixed for all affected courses and users but it seems to be happening again - managers see reports with learners missing. Trying to recreate the scenario and test it is very time consuming so I wonder if anyone else has had similar problems and may shed some light?