Lead melting pots require cleaning, and this is especially true of bottom pour pots. The alloy components oxidize and the small particles of oxide aggregate and adhere to any surface. This dross coating builds up on the walls of the pot and on the valve components, and slowly clogs the nozzle. The dross forms an insulating layer and this means that the heating element has to run hotter and longer. If the thermostat is external to the pot, as it is on most pots, it means that the actual temperature of the alloy at any setting slowly decreases. The cure is to stop and clean the pot. I usually clean my RCBS pot about every 10 hours of casting. This time I was lazy and about twice that, had elapsed and I was reminded of my sloth when the nozzle started dripping and I was getting lots of reject bullets with dross inclusions.
I drained off most of the alloy and turned the pot upside down while hot to get out the very last drops. I clean with a sharp but blunt tip knife, shaving the walls and scraping the bottom of the pot. Here is a picture of the quarter pound of dross I removed. Pretty ugly, I waited too long.