I live in warm sunny SoCal, but I still heat the hook and collar area on 90% of the jigs I pour.
Air trapped in molds will only give you a minute incomplete fill.
If you are heating your hardware and still getting bad pours it's a lead flow problem.
Clean your spout. Wear gloves and ream out the spout with a piece of wire. (spinnerbait wire form)
Be careful!
Also placing the lead nozzle all the way down into the pour hole can prevent air escaping from the top.
Gravity is your friend. I like the nozzle about 1/4" above the mold. Everyone has there own way of doing things.
So do it anyway you want, I am just trying to help. Good Luck.