Sorry, I did not read carefully.
Media can be offline if Vegas can’t open it. Then there’s a “media offline” text on the timeline, but this can be covered by other items depending on timilene zoom, so it may be unnoticed.
Just few more hints:
-the events are not muted accidentally? (Not at track level, but at event level – switches/mute)
-opacity of the venets are not 0% accidentally?
Best regards
László Kovács