There are lots of tests the doctors use - MRI, X-Rays, CT Scan, myelogram, discogram, etc., and each one has something they tend to show better than other tests.
Myelograms are really good at showing if there is any compression of the nerve roots, and yes, sometimes they show problems that MRI's or other tests may miss.
Be sure to do a search here on myelograms - lots of information and tips in the old posts about this test.