I use straight wheelweights and dump them right into a 5 gallon bucket of water. If you want to go fast, this will harden the bullets to around 18 bhn.
I dont let the bullets stay in the mold for very long. As soon as the lead solidifys I cut the sprue and dump em out .
Check your sprue plate. You may need to take it off and stone the back of it or it may need to be cleaned.
Also, you might need to use a different method of whacking. I use a leather mallet that is heavy enough for one whack. I did use a wooden handle that required a lot more effort and usually took several whacks.