The reason for the varied responses is that there are several sub-types of NMDA receptors. Each NMDA (and many other) receptor is an amalgam of sub-units, and the types of sub-units involved change the structure, and thus the response to different chemicals. To make it worse, many agonists (ie. chemicals that'll bind to the receptor and cause it to open) aren't perfectly selective to one combination of sub-units, a fact that made my doctoral research partocularly fun. So if a drug is designed to target one type of NMDA receptors, often, it'll have off target effects, which in some cases can be beneficial.

To complicate things further, there's evidence that some types of sub-units are produced less in some brain regions in major depression.

On a tangent, there's quite a bit of work linking autoimmune conditions and depression. I'm pretty sure you've come across this, but if you need some literature, let me know.