I have imported a Maya model into Unity successfully. Then I go back into Maya and update the model. The changes do not appear back in Unity.
That is because the files are not linked. The file you have in Unity is going to be the same as what you exported it as. If you want to see your updated model. Export it again, import the new model, and delete the old one.
You probably have to have the maya file inside of the projects directory structure. I think if you import an asset, it just copies to the projects dir. structure (i think, not sure)
Also, Unity might turn off automatic importing if somethings crashes.
I was thinking something like that. Maybe because I made the Maya object, then opened Unity and imported the object as an asset. It shows up in the asset folder now but maybe should have been placed in that folder from the start?