Have you tried pushing forward under the trim? Years ago I saw something similar and wtched the tech push siding under the trim. They don't cut that stuff perfectly because they know the trim will cover up the ends. It may have slid so far to expose the one end.
If it does side forward I'd suggest you tack it down with some caulking before you slide it in all the way. That way you won't see the caulk. Otherwise you'll probably just need to caulk it where it sits.
2004 Cardinal 33CKT
2006 Ford SDD F250 CC 4WD