Yeah, it's pretty lame having a good room going but people start leaving due to boredom of the same track. So you explain you'll make a new room for a different track, and only 2 or 3 follow you. You have to wait till non friends from the previous one find you, or get a new lineup of racers join who just want to race dirty.
This needs addressing ASAP as it's killing the online experience for me and others. You could change tracks from custom to original, and back again in GT5 as many times as you liked, why not in GT6 ?