Just some ideas. It doesn't sound like a magazine problem, other than the fact that it is the last round. Since guns can't count the only thing that your CM knows is that this round has less spring pressure to force it up and out. I would check the extractor and maybe polish/radius the bottom edge and also check the tension on it. A simple way to get it close with no special tools is to remove the slide and barrel and take an empty case and slide it under the extractor it should be just tight enough to hold it in place but not so tight that it is hard to get in. To loose and you can get extraction/ejection problems, to tight and you can also get feeding problems particularly on the last rounds when mag spring force is the least.
Hope this helps!!
Let me know!!
L8r
Conley