Geoff-
I'd agree with Shelley that your root symbols are likely inadvertently comprised of multiple classes.
An easy fix if that is the issue rather than editing/checking each symbol try this:
Create a new class...i.e 'lighting instruments'
Group all your lighting symbols together in one folder and use the Tools/Utilities/Change Symbol Atts to make all your root symbols now in the 'lighting instruments' class. Hopefully, that should solve the problem of symbols being comprised of different classes.
Tmonson