New Zealand air gunners without the BoB clasp

Hi all,
I notice that four New Zealand air gunners who were with 264 Squadron during the Battle of Britain failed to receive the BoB clasp.
I understand that in order to obtain the clasp, aircrew had to perform an operational sortie during the required period. what I cannot understand is how four qualified air gunners, attached to an operational squadron during the Battle of Britain (especially a defiant Squadron) failed to notch up a single operational sortie.

The four in question are:-
William Louis Butler – on 264 16th Sept 40.
Leonard Roy Preston – on 264 16th Sept 40
Ivan Norton Robinson – on 264 27th July 40
Bernard Cyril Simmonds – on 264 16th Sept 40

I would appreciate any help that you may be able to offer in clearing up this particular query.
Gerry