Auto import has been inconsistent for me across at least three versions (24, 26 & 28). Sometimes it imports immediately, other times it will simply not pick up a new file. I have to do a manual import or restart and then the file is immediately imported. There is no consistency regarding file types. It does seem to happen more often the longer the software has been open, i.e. it doesn't tend to happen when I switch on in the morning, far more likely later in the day.
I should add that my files are on an internal HD in the same PC as the MC install.