Hmm then i'm afraid i am out of ideas, the only thing i could suggest now is to delete the platform and re-import using Retroarch as the Emulator from the start (you would need to check the force importing this system as MAME on the last window of the wizard to make sure it imports as arcade), this will be quite a quick process as i assume you already have all the images and such, so you can uncheck all of them in the wizard, so it should only take a few minutes to import this time.