BlackPowderBen -
You are probably going to get a variety of answers based on the experiences some have had. Some really "get in to it" when casting and are quite "high tech" . . others? . . not so much. I'm a "low tech caster" and have been for 50+ years.
If you or anybody else wants more information on casting, etc. . . .I recommend you go over to the castboolit site. Doesn't cost anything to join and there are not only "sticks" on what you are looking for but lot's of good reading and information the the various threads there.
In a nutshell . . thee are a variety of ways to prepare a mold depending on who you talk to . . . plus a few more things in terms of the actually "casting". Alloy you use (for BP you are using pure lead), getting you mold up to temperature, the cadence of your casting, etc. It's not "rocket science" though and anybody can learn to cast good balls/bullets.
You obviously have been casting so you know what's involved. As far as not filling out . . . if you mold is up to temperature, how about the heat of the lead you are pouring? Also, are you using a ladle or a bottom pour? I probably have about 75 or so molds in my collection that I use for not only RB but conical for reloading cartridges, etc. A wide variety of brands and materials - iron/steel, aluminum, brass. I have never had an issue with the size of any sprue hole as far as cavity fill out . . . but that is with my molds and there are those that i know that have had to alter the sprue plate. I can tell you though that each mold is a creature unto itself. I have some that drop like butter . . and a few that don't!
You don[t mention the size of your cavity you are pouring or if you are using a ladle or a bottom pour pot. I only use a bottom pour Lyman ladle - same one I started out with fifty some years ago. I pour over my main pot (on a gas hotplate) and always fill my ladle full - any "over run" runs off back into the pot - but that puts some pressure on the cavity fill. The largest I pour is a 500 + grain minie ball and I find the above to be the best to pour it and insure good even "fill". Smaller cavities such as a 90 grain or a 158 grain are a cinch - but I'm only using 2 cavity molds. I don't have any 6 cavity molds as I know that keeping the blocks evenly heated and a continuous pour of all six cavities is probably going to be a problem unless I had a bottom pour pot.
Not much help I know, but at least go check out the castboolit site as I think you'll find it an interesting site to look at. Good luck!