I had to see several doctors before I got help with my double vision. Many ophthalmologists are not experienced at prescribing prism. As Dr. O suggested, you could see either a neuro-ophthalmologist or a strabismus (eye muscle) specialist. You can find one in your area using the doctor directory at www.aao.org. I suspect that your general ophthalmologist has already done what he can for you, and you need to see a specialist.
see previous answer.
Dr. O.