Could be any number of things, wyodeacon - but probably it just needs yet more practice!
Landing is the most difficult part of FS - actually considerably more difficult than landing the real thing.
Brett-Henderson's advice - to look at the whole runway, not just the threshold - is very important.
The other key thing is to relax - it takes some minutes to carry out any approach, take your time, don't use up all your energy trying to stay dead in line from five miles out, it's much easier to straighten up once you're closer in.
Speed is another issue - IMO the approach speeds recommended for various aircraft in FS are a bit under-stated, it often pays to come in maybe 10 knots above the recommended speeds, gives you more 'oomph' so the controls answer more positively. You can always bleed the extra speed off in the flare.
To help your practice, I'd suggest saving an approach say 15 miles out and using it for practice. You'll find that you can substitute any other aeroplane 'in flight', helps a lot when you try out a new type.
If there's any sort of wind you may need to 'crab', aim to one side of the runway, otherwise you'll keep drfiting off line.
Also see if either of these help:-
http://www.simviation.com/cgi-bin/yabb/YaBB.cgi?board=COF;action=display;num=1111322151http://www.simviation.com/cgi-bin/yabb/YaBB.cgi?board=COF;action=display;num=1129643666