I would check the face of the mold(s) be sure the air bleed out surface is clean maybe you are getting a little back pressure there
and the drive band is not filling?
I also think if you slow down your casting a Bit it will give the melt time to fill nooks(drive band) and crannies
I don't have
RCBS bottom pour I have a Lee Bottom pour,It has a temp control.
If your does also, I think you can hold off on the thermometor ,If you have the money get it, I think you can just try and adjust the dialsave the cash for more molds
I believe if you have a Frosty Bullet as was posted that is from Too HOT the bullets should look like your Ingot
DO you Flux after your Pot melts and when you add more too it ?
Tommyt
Psss
I just got a thought Do you Keep your Mold tilted as you Pour? so that the Melt rolls into the mold try different angles
when you get a full Mold/Bullet, Repeat with the same Pitch .
I bet that is what has been happening to you.