With a very low level of B12 for a long time period... it may take a while to recover. If there has been permanent damage to the spinal cord from the low levels, there may not be 100% resolution.

This post link has a copy of the paper by Dr. Snow, which is from 1999. His take which I bolded there is that patients with neurological problems may have to be kept at higher than typical range for treatment, than most others:

http://neurotalk.psychcentral.com/post698522-70.html

The cut off in US now appears to be 400. But following Dr. Snow's advice, would mean you should strive for higher. I'd say at least 1000. You can reach that goal by taking oral properly.
5mg of methylcobalamin (which is the active form) daily on an empty stomach has worked for many on this forum.

Many doctors don't know about active form, being available and so hence they still use the synthetic which some people cannot activate in the body. That link above I gave, is from the B12 thread, which has medical links and explanations about the various treatments and reasons for them.

So please read it carefully, the whole thread, and perhaps then your path may be clearer.