Dave, I suspect the vent lines are not deep enough. Sounds like a little work deepening them would give you better fill-out. it's not rally hard to do, but you have to be careful NOT to raise any burrs on the edges of the cavity. An engraving tool is best for this, but any steel with an edge that can be pushed or drawn along the vent lines will work.
Another thing is sometimes the sprue plate is too tight to the top of the mold, it has to have a tiny bit of a gap to vent the top of the mold also.
The more people I meet, the more I love my dog
They're going to get their butts kicked over there this election. How come people can't spell and use words correctly?