There are a few reasons your cast bullets are oversize. 1) There is a nick or burr on the mold face 2) Splash of lead on the mold face 3) No or insufficient lubrication on the mold alignment pins.
A common error with bullet molds is omission of lube on the sprue hold down bushing. This makes it difficult to swing the sprue plate all the way over to the "closed" position in preparation for the next cast, and also tends to force the mold block halves apart, resulting in out of round, oversized bullets. If your bullets are oversized when measured perpendicular to the part line there is a very good chance the mold is not closing all the way due to the lack of lubrication.
Aluminum molds require occasional lubrication. They should be re-lubricated if/when you notice the sprue plate begins to become more difficult to move and/or when the mold blocks do not line up when the mold is closed. Lack of lubrication will cause the mold blocks to mis-align, or the sprue plate to gall the top surface of the mold.
A technique that works well is to have a glob of hard, stick-type bullet lube (like beeswax or Permatex Anti-seize) about the size of a .45 slug rolled into a football; when the mold gets hot, touch one end to the aligning grooves along the sides of the mold block, on the underside of the sprue plate, and on the steel pins along the bottom of the mold block. Use sparingly, as if any gets into the mold cavity it will cause wrinkled bullets.
