Ocular myasthenia gravis. Double vision.

Hi, I'm having a progressive diplopia, maximum when i look down, both left and right. I have proper vision if I close any one eye. My vision is better sometimes, i mean intensity of diplopia varies across the day. I met neurolgist and nothing could be found from contrast brain MRI and MRV. The CSF study was also normal. ANA and thyroid profile also normal. Opthalmologists couldn't find anything wrong with eyes. Suspecting ocular myasthenia these tests were done. RNS was negative. AcHR and Anti Musk antibodies test were negative. I don't have ptosis. Keeping ice pack on eyelids didn't improve my double vision. Pyridostigmine 60mg 3 times a day didn't improve diplopia. I have no other issues, no fatigue / headache / pain anywhere. Never met with an accident or a stressful situation. Not diabetic. I feel healthy and I run everyday. I weigh 70kg with a height of 183cms. With all these tests negative can I still have Ocular myasthenia. What can be other causes for my double vision.