Have the same pot same problem occasionally. Run your temp a little higher. That's why its plugging. Most forget that a little part of the pots nozzle is out in the open air. And most of its orifice is wide open to the air also. Don't drill. You'll oversize it's orifice or make it passage out of round and eventually have a leaking pot with and un-adjustable pot valve to stop its trickle flow.
Kudos to dahermit and his suggestion.
Been doing the Boric Acid thing many years myself. Reason to do: I don't like stirring a piece of wax at all into hot dross to make a good flux. So if I can do it one time for the entire pot full I'm happy. As long you don't break the surface tension of a Boric Acid sprinkle. You'll be able to draw that full pot down to nearly empty without having to flux over & over because of its tin separations. (that means don't throw your molds sprue cuts back into the pot) Just flux it once early after melting. And reasonably quick sprinkle some Boric Acid over its entire surface. Tip: Just a little bit of Boric Acid goes a long ways. I use a salt shaker to do it for me. A simple but very effective way to cut your fluxing down.
Watch it!!! their both out to get us.