AF works by phase matching, in others its like overlapping 2 lines, once the 2 lines overlapped exactly they are in focus.
thats why patterns can cause AF to hunt or mis-focus even under broad day light.(same pattern means hard to determine which pattern overlap whcih)
back to the focusing problem, try finding strong lines along the same plane of focus and ths will allow the focus to lock. (e.g the outline of the ear cos it usually stands out quite nicely against the dark back ground for focus.)
festures of the face under low light has too little contrast for the camera to detect.
AF works under a certain range of EV( light values). eg EV1 -21. means brighter lens will work better cos allows more light in and thus easier to get at least EV1
more importantly to work around the problem be focusing on strong lines then getting brighter and brighter lenses. 2.8 is already pretty good for most situation even outdoors at night.
just my 1 ct worth