Make sure you are dealing with good quality lead/alloy. If wheel weights, melted zinc will cause issues, make sure you only use lead wheel weights (getting rare these days).
If lead is good quality, make sure you flux well through the process. Fine saw dust, commercial flux, all work to draw off the dross and impurities.
Once lead is hot mold temp is important. So is technique for mould fill out. Watch the bottom of the sprue plate, if you develop lead streaks as the mould heats up, wait longer to cut the sprue.
A hot plate is beneficial for mould heating as well. If stuff starts coming out frosty, mould is too hot.
It's an art that requires consistent technique. But can result in good quality bullets with good quality control. But it takes practice.
Check out castboolits too. Great info over there.