I just dealt with this for the first time about 7 mins ago...lol I learned that if the nozzle is too cold, filament won't come out, and if it's too hot, it will likely leak out even if you aren't printing anything. So I think there is a sweet spot for each filament. And I also had to give a little push of my filament line into the extruder too to make sure it was in all the way. If that advice sucks, please someone let me know. Hope it helps!