You are casting with either or both the mold and your shank alloy too cold, and or, not giving the softnose time to come up to mold temperature. Everything MUST be hot, because the shank alloy actually melts the surface of the softnose and welds into one piece when .
Most lead pots must be run at max heat. 900 deg is good.
After placing the softnose (s) in the mold, dip the forward lower corner of your mold into the melt for a few seconds to heat the mold hotter, then make you cast. The instructions which came with your softnose mold spell all this out in detail.
