The knock spell does not work on plot chests and / or some other items marked "plot". This is a function of the base game, tied to these chests ability to not be destroyed by environmental effects. A caster with only Knock will not be able to open any/all chests on the server due to this, though it does work on some.
The open lock skill is currently the only reliable way to open locked chests and doors, usually requiring at least 1 skill rank to open nearly all chests.