I've seen this type of thing happen when the taxi link to
the runway is not complete. I had that happen at Hobby on one
of the runways.. The planes would taxi fairly close to the
beginning of the runway, but couldn't finish access the runway,
so they would just sit there and block traffic..
It's been so long, I forgot exactly what I had to do to fix it,
but it was a "link" of sorts from the taxiway to the runway.
It was missing, so the planes couldn't finish the taxi to the runway.
This will happen in FSX too, as the traffic setup is basically
the same.