I've never had an issue like that when casting them, or bullets, or ingots. If I had to guess I'd say you might try to increase the heat some and get the molten lead hotter. I like to see it solidify at the top of the mold a few seconds after filling. Get you some pine tree rosin and flux the molten lead with that, too, before casting. It'll bring more dross to the top.

SRH

May God bless America and those who defend her.