Charley, If you casting with pure lead or something close, high casting temps will cause the alloy to turn blue in color so when you add alloy and flux you are lowering the temp which makes things right again.
Lower the casting temp and see if that will help.
Some beleave that lead alloys will seperate (no flames intended) but that's not ture. What looks like seperation is oxidation of the surface with oxygen in the air.
If you are using a bottom pour pot you can elimanate oxidation by putting a 1/4 in. layer of the cheapest clay cat litter you can find at the G store on top of the melt. It also helps hold in heat.
The next time you fire up the pot, flux good, add cat litter, you will never have to flux again. When you add sprus in just lay then on top of the CL they will melt and run thru fluxing along the way.
The CL will break down and turn to dust aftet 4-5 casting sessions, just scoop it off and throw it in a coffee can to cool, add new and discard the old.
If you are ladle casting, forget the CL and flux every 30-45 min.
Calamity Jake