Severe upper back pain progressing over months, left arm weakness, worse at night- clear t-spine mri w/o contrast- could it have missed something?

Assessment: probably needs further evaluation considering the chronic its of the problem. A full physical examination with a physiatrist in collaboration with your PCP plus consider an MRI of the cervical spine if not done in light of the upper arm weakness.

It is possible: It is possible that the MRI did not show a nerve problem causing pain and weakness. You may need to see a neurologist for evaluation and possibly nerve conduction studies. Good luck

C spine problem?: Has your cervical spine been checked? Left arm symptoms are much more commonly related to cervical spine disc and degenerative problems than thoracic problems. Get a good exam of the cervical spine and left shoulder. You may need a cervical MRI and NCV study to localized etiology.