Were those first 100 bullets nice and pretty, or were they wrinkled? If the mold was still contaminated with oil, as they always are when you first get them, the oil might have acted as a release agent.
Or maybe you are using a different alloy now that doesn't shrink as much.
(or maybe the bullet mold is just messin' with you)
"The only way to stop a bad guy with a gun is with a good guy with a gun"