what are you using to flux with ???
I use parrafin wax or a DRY stick , anything to induce carbon into the melt !
Plese don`t melt your dirty wheel weights & shot(recovered??) in the Lee pot , do it in a seperate steel or iron pot & ingotize it !
Alot less chance of causing problems in the melter !!
In the melter I do a last fluxing & leave it on top to create a O2 barrier , but pull it off before adding sprues back to the pot .
PS: if ya try parrafin be advised it either smokes alot or will ignite !!