I agree with what's been said here already that the most likely reasons you're not getting complete pours are first, the mold may not be hot enough, second there could be a venting problem, and third, your bottom pour is not providing enough flow to fill the cavities quickly enough. It's possible you have more than one issue.
Many folks with such problems presume their mold is plenty hot, but if you consider that the mold may be for example, 400 degrees (F), and the melting point of lead is slightly over 621, that's more than a 50% differential (221 degrees). Lead can't stay molten at 400 degrees, or even at 500, so it's cooling before the cavity is filled. As was mentioned a thick mold will cool quickly as heat is dissipated and will be slow to heat up enough. The mold temp needs to be closer to that 621+ temperature. I use an electric hot plate to heat up molds and keep them hot between pours. That fixed most problems that I had with incomplete pours, provided I give enough time to get the mold as hot as it needs to be.
Some may tell you the lead needs to be hotter, and although that would help, is not the real problem. If the lead is molten, then it's hot enough. Pouring a molten metal into a mold is about heat transfer and balancing the heat energy. The heat will transfer from the molten metal to the mold, but the metal still needs to stay in the molten form long enough to flow & fill the entire cavity. If not, then you get incomplete pours.
Even if the mold is hot enough, it doesn't appear that your mold is vented. CNC molds with tight tolerances won't allow the air to escape the cavity fast enough. Vents do not have to be large, just enough to allow the air to escape. If you clamp the mold, that makes the problem worse.
For pours that are an ounce or less a bottom pour should have enough flow capacity, but if all of these issue exist, then a flow issue would be compounded. I have both a bottom pour and a Hot Pot, and prefer the Hot Pot for any larger pours over 1 ounce. The only time I have incomplete pour problems is when I let the mold cool too much. All molds that I have that did not arrive with vents, and closed well, I added vents.
Those of us with some of the older molds that were cast instead on machined, and did not close very well, likely never gave venting much thought because there was enough gaps to let the air out. Most of the Do It molds now are made with vents, but some of their older molds may not have had them.
Much of the pouring I do, is with recycled wheel weight lead, so on the hard side, and I still rarely have any problems. For your issue, I would first start with making sure the mold is hot enough, then go for there.