Start off by cleaning the mold, if the mold has any oil in the cavities you have a problem. Don't be afraid to get it wet give it a good scrubbing with a high quality degreaser like "Oil Eater" and a toothbrush then scrub rinse in hot tap water. If you are still worried an acetone scrub won't hurt.
Once the mold is contaminate free, turn the pot up to 800+ degrees and confirm that the alloy has reached that temp with a thermometer. Preheating the mold on a hot plate helps to bring it up to temp if you have one, if not start casting letting the mold rest between pours to absorb the heat in the blocks. It may take you a number of pours, but you should be dropping completely filled out bullets. Once you find the sweet spot temperature wise adjust to your liking. Brass molds like to run hot and especially molds with pins. Give the mold what it wants and it will give you beautiful bullets.