Sharpshooter - Semi is right. I usually cast the first dozen or so and put them right back into the pot. Keep the lead hot. Once I start getting bullets without wrinkles, I cast as fast as I can.
I used to smoke the mould cavity so the bullets/balls wouldn't stick but I found an even better trick. I put a piece of bees wax about 1/2" square in the molten lead and after the smoke clears, I scrape off the black residue and start casting. The bullets do not stick to the mould. I've heard that others use PAM but I haven't tried that yet.
As for the weight of the balls being lighter, Semi was right again, your sinkers were probably not pure lead. And for a smoothbore, it probably won't be a problem.