Not sure what the alloy your pouring is, but if it is lacking in tin this could be causing the fillout issue. Not hard to determine, just add in some 95-5 solder into your next pot to sweeten it up. Be sure you weigh it as well so you will know about what is needed next time if it helps out. Just measureing the length will work as well.
With the WW alloy I have been using it takes about 6-8" worth in a 20# pot to get there. I started out with a full pot, then I added it in 2" at a time and stirred real good, then poured a few bullets. I kept the temp set on about 700, and the mold good and warm sitting on the hot plate while I mixed. Sure enough when the tin hit the right amount nice filled out bullets with nice sharp rings on them.
Hope this helps.