i think everyones got it, mould or lead isnt hot enough, Try dipping the corner of the lee mould in your melt until its warm enough lead wont stick to it. if that doesnt help try running your melt hotter, that way it will take longer to cool and give the ball more time to fill out.
experiment with pouring technique too, Both my lee and lyman molds like to be poured fairly quickly with a good puddle left on the sprue, and in the case of the lee, ive found keeping my dipper in contact with the sprue plate helps the mould hold the heat a bit longer giving the ball a chance to fill out more completely.
As a last resort, try some different lead if you can, i experienced this with a few lbs i acquired, that had been melted down from shot. Not only was it extremely dirty, i assume from the graphite used as lube, i like to never got it cleaned up, but it seemed to be more prone to casting imperfect balls from both moulds. After i got that batch of lead gone, and moved on to some other ingots ive got, the problem went away as long as everything was up to the proper temps.
