It's probably a combination of things. When I'm casting the first 10 or so castings are scrapped as the mould gets up to temp (Lee alloy moulds) and then it's just a case of getting into a rhythm and casting steadily, too slow and the mould cools, too fast and the bullets are still part molten when you are knocking them out.
You don't mention what you are melting the lead in, if it's a Lee bottom pour jobby, use a ladle instead of the pouring spout.
Are you fluxing the lead too? I use beeswax from a colleague who keeps bees.
Then practice, practice and do a bit more practice.
Rich