It is not rocket science. If a mold is not closing all the way, something is keeping it from closing. Open the sprue plate and hold the mold up to the light to see if light is shining through where the halves meet. After a careful examination you have
not found what it holding the mold open, you should go to your eye doctor.
In short, examine the mold carefully, something is holding it open, be it a speck of lead or a burr on the Aluminum itself, or lead has entered, filled the steel pin alignment hole(s). There is a
small number of things that can keep a mold from closing all the way. If you look, you will find it.
If you cannot find anything that is holding the blocks apart, it is usually some lead that has gotten into the hole for the alignment pins...a frequent occurrence if a person preheats their mold on top of a casting furnace and the mold slips into the pot.