I would suggest seeing your dentist. The dentist can devise a splint that will prevent you from grinding your teeth at night, which may be all that you need to alleviate the pain and stiffness in your neck and shoulders.
I'm afraid I can't tell you whether or not continuing to see a chiropractor will be helpful. But if you've seen no improvement after 12 treatments, you might want to wait and see if the dental splints will help before making your decision regarding chiropractic treatment.
Good luck to you.