Cops like to crack down every now and then to try and prove that they are actually making progress in addressing the "problem" of the average teenage marijuana smoking criminal. So whenever there is a major bust people have been hearing about, the dealers are probably just laying low for a while, which will explain any temporary dry spell. The drug trade is simply too large to ever be stopped, as even if you take down one druglord, another one will just arise.

Even if a huge source is destroyed, there are always others so its only a matter of time until the dealers get their hands on another source. Its almost incredible how the economy of the drug trade works.