Hi
I had the same problems a while back,
I always place a waypoint a considerable distance from the runway
I usually make it like 1000 meters. in most cases it's south of the runway.
put the planename land "LAND"; command on this waypoint
this way the plane has enough room to make it's approach.
with the exeption of some addon aircraft I never have problems with making AI planes land any more.
hope it helps
MadFred