strange. but i had problems like this, when bone influences where to far away for mayas auto-weight. but i could edit it via the component editor.. try this: select both vertices and then go to the component editor. now you might be able to add an influence value for that unweighted vert. or try the weight hammer on that…
try to select 2 vertices. in the component editor the normal vertex should show all the joints. now pick one of the joints and type 1.0 into the column for your problem vertex. that should have some effect and assign weight to your problem vertex. does that work?