If you are frustrated by some aircraft not plotting then have a google search for planeplotter. This is not free, although the one off fee is small I believe. It uses a bot of wizadry called multilateration. Its nigh on rocket science how it works (or so it seems to me) but it results in non-plotting aircraft plotting. I believe it does this by comparing time differences between receiving stations and doing some thingimajikery to calculate an aircrafts position.
Regards
Steve