I don't get sore any more — why isn't my weight training working?

Soreness is not an indication of anything in training unless it is bad soreness, in which case you have injury. You can just as easily haved damaged tissue in need of repair without feeling sore at all, because the prevalent theory regarding the soreness is inflammation caused by amino acids leaking into the tissue. So just because you feel like you could train again with a heavy routine, doesn't mean it would be practical. Your central nervous system gets taxed any time you train, and in some cases (this has been researched, so this isn't guesswork here - it's actually scientific data) intense training can take as long as 11 - 14 days for your body to fully recover from!