I’d almost bet money that the temp is reading incorrect. Just try and bump it up to say 825 and see if it’s better! 
Won’t hurt a thing to try it.
Should help!
Sst

When all else fails, smoke the mold holding it over a candle flame and / or boil it in water for about 10 minutes. 

There is a slim possibility you may have antimony contamination  (correction: it was ZINC contamination)  in the alloy.  You will never get a filled out bullet if you do.

The mold cannot be at melt temp.  It will take 5 minutes for the bullet to harden. All my mold temps ran between 410-420 F for the .32/.33 to 450 F for the .45s. Weight variance trouble starts at 399 F mold temp    Not sure about fill out temp, but certainly lower.   Melts ran 750 to the low 800s.  The Lyman thermometer is a fuzz high, so I'm making allowance to approximate  the melt temp, but the mold temps are dead on with my Fluke thermocouple. 

If you don't have a mold temp device, with a preheated mold, you should be up to casting temp within the first 20 bullets at a consistent pace. With the temp listed above my bullets stay within +/- .2 gr.   I usually have to slow down or wait a minute or so once in a while to keep the mold from getting too hot  to hold the weigh accuracy. 

One other thing you can try
You may not be getting enough pressure. If you are ladle pouring tru opening up the hole in your dipper 1 or 2 drill sizes.

40 Rod

The best mold cleaner I have used is Micro 90, 5% solution of the concentrate
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Use a business name and order a free sample
Heat the mold with hot tap water - scrub wit a tooth brush - rinse and heat to temperature. Cast with a bare mold, no coatings

CW, Schuetzenmeister,
Indeed, It has been said that Metallurgically it is Not so Good if tin is in a greater percentage than Antimony in a three component alloy.
So, perhaps if antimony is present in a low "impurity" 
amount in a basically Lead/tin alloy, it may interfere with 
fillout.
OTOH, in proper amounts, With Antimony in larger amounts
and in equal or greater amount than Tin, we get perfect fillout:
like for example one of the Best alloys for Excellent bullet
fillout,is namely Linotype- Designed for that purpose of perfect fillout of tiny type set. It is 82%lead/12%Antimony/6% tin.
Of course Lino is too hard (at least Traditionally) for making
Schuetzen bullets , especially for breech seating.
A prolific Engineer and experimenter in cast bullets, 
Dan Theodore came up with a very interesting alloy
that works very similar to Traditional Lead/tin alloys:
97%Lead/1.5%Antimony/1.5% tin.  Very close to 16+1
which is very popular for some of the BPCR bullets that have elongated noses-Elliptical, "Money",etc.
beltfed/arnie

hepburnman wrote on Mar 14^th, 2020 at 9:06pm:


I have had this problem. I can often tease a troublesome mold into working but I remember twice when working with new to me (used) molds that nothing worked. The problem was just as you described and turning up the pot heat nor slowing down the ladle pour failed. Always, the one side had slightly rounded corners and was not filled to nice sharp corners.

As a test I awkwardly switched hands and poured turning (rotating) the other direction and the bullets were filled on the offending side. I concluded the liquid metal was swirling into the mold in some way that was hindering the air escape on the unfilled side.

If cleaning the mold has already failed, check the vents. 
Then, play with a hot(er) mold. I place the mold blocks on the pot edge at the start of a cold pot, handles on a side board the height of the pot so the mold stays put and level. Cover it and the top of the pot with a square of aluminum foil. Stick the ladle in there too while you are at it.  45 minutes later, if your metal is truly 725 to 750F, I bet it pours good bullets.  Mine will often pour a good bullet from the very first one out of the mold. 
That is my best guess with out being there.  

If you give it a try I would like to hear how it goes.
-CW