I personally have never experienced any difference with humidty. If you are pouring outside with lower temps, or with a low pot you can experience this. I would make sure the pot is screaming hot, Mold on top getting screaming hot, full pot of lead on high, clean the nipple with a spinnerbait wire, paper clip etc. Smoke the mold with a candle and put the nipple right into the pour hole and "inject" that sucker with lead.
Thats what I do. Lead Molding Basics - Casting problem diagnostic guide