Hi James I have only done this once so may be wrong
It is clipped on the non swivel side but screwed in at the swivel there is a little square cover over where the screw should be, have you taken this out to ensure the screw or the fitting it goes into is intact?
I can't remember is the screw expands the plastic to retain it or screws into part of the roof but once removed all should be obvious to you
HTH
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