How are you filling the mold? Ladle? Bottom pour pot? Pure lead or wheel-weight alloy?
If everything is hot I'd start with the filling of the mold. The first thing to try is free....try tipping the mold towards the hinge side about 15 degrees, maybe 20 as you fill the cavities. If you are using an alloyed lead, go to pure lead. If you are using a bottom pour pot, try dipping a 3 ounce ladle so the lead stays very hot during the pour.
I have the vibrating "sonar" type of mold and in one cavity I had to create an extra vent using a small triangular file.