Why can't I a specific category to my existing listing on Motors ?

[corbica]

I have an existing Motors Classified listing and would like to add another category.  The category I wish to add is  Motors > Cars, Motorcycles & Vehicles > Classic Cars > BMW [which exists] but it does not offer me anything under the Motors > Cars, Motorcycles & Vehicles directory tree.  It offers me lots of others under Vehicles etc... which isn't what I'm looking for.  Are there certain category trees that can't be added after a classified listing has been done or something ?

[fargvs]

Once a Classified Ad. is accepted, posted and goes 'live', no changes can be made to the listing.

[corbica]

I can make other changes to the listing e.g. the price etc and description etc... Are you saying that I just can't make changes to this ?