If ya let the melt sit & cool to say 600-650f does it have a bit of oatmeal consistentcy on top ????
If it does you could be fighting zinc contamination !
I would get some of that leadfree solder & add a 8" peice to 1# of melt & see if it helps if it does carry on , if not pour ya some fishing weights for ya buddies .
Ya can go to the big builders stores & buy it by the lb., I can remember when it first came out "leadfree" for $6.00 !!
If ha have some ingots that have stayed extra shiney there hi in tin & antimony & should help a tin deplited melt.
Good luck !!