I bet ya add sprue cuttings as ya go ??? !!
They pull the oxidized top layer of tin into the melt & dross , dirt or whatever is floating on top .
Each time ya add something to the pot flux & stir , I personally like parraffin wax for fluxing in the casting pot , be aware that it`ll ignite on it`s own & make ya jump a little ( not good with a spoon full of hot lead).
After fluxing I sprinkle a little sawdust over the top , hard to do on a ladle job though .
& pour a little slower if ya pour directly in the sprue plate hole so air can have time to escape.
Of the 4 bullets ya have in the pic I mite cull the 1 with the imperfect lower band the rest are shooters .