With regards to human error, my most common mistake in this regard is to miss an edge that is more or less invisible from the view that I am currently using. To fix this, zoom around with the perspective camera to make sure you've included all of the little fiddly ones.
Once in a blue moon, however, the program won't fill in a hole with no apparent reason. Going through the checklist: all edges selected, drag select/merge points at all of the vertices, etc. still nothing. If this does happen, and I must point out that is rare, there are a couple of quick workarounds: first, try making an edge across where you want the face to go, fill the faces for each half and then merge the two faces together; and if that doesn't work, cut one of the edges, draw an edge from that new vertex such that you remove one triangle from the face, fill the rest, move your newest vertex over to cover what's left, and merge points. The second one is rather difficult to explain, but actually easy to do. I hope that works, I really don't see it often enough to be able to tell what causes it.
