I cast a variety of lead jigs and weights. Off hand, I would say that temperature is the culprit.
I use commercial diecast molds that have material removed from the outside to reduce loss of heat. When I pour, I will fill the outside cavities with the molten lead several times before attempting to pout the mold. A higher melt temperature will help as well, Also, fluxing the lead and removing dross prior to pouring. I have seen but do not use some pouring ladles that have a gate at the bottom to prevent dross from entering the mold.
If I were looking at mass production, I would redesign the mold for faster openng and closing. My molds are all hinged and I use a clip to hold them closed while pouring. To clear a mold, I pop the clip, open the mold and give it a light tap to discharge the castings. I would also make the reservoir at the top of each cavity bigger. I milled my molds out to make a common trough so I dont have to hesitate when moving from one cavity to the next.
As others have pointed out, you may need to add a vent to each cavity. It won't take much; a scratch at the bottom of each cavity would do it