There are a whole lot more physical processes going on in our bodies then just neural activity. And my best guess is that is exactly where reductionism fails. It is possible that neural activity is a necessary but not sufficient condition for consciousness. It is also possible that we are looking in the wrong direction, that consciousness arises via interactions with the world. In either case (of which I find the former quite convincing) we will never be able to describe the mind by just looking at neural activity.
I am actually of the opinion that cognitive scientists are doing an excellent job describing the mind with our current theoretical models which excludes the tough questions of consciousness.
It might not even be possible to fully understand the physical mechanisms that underlie consciousness, but that doesn't mean there has to be something more than physical mechanisms.