Sometimes it possible that you can over flux and remove too much of the TIN in your alloy and cause fill out problems and as mentioned it could also be trash in your melt.
Not possible! In fact, the reason we flux is to RETURN tin to the molten alloy, along with lead and antimony that have oxidized. But because tin melts at a much lower temp, it oxidizes at a much higher rate than lead or antimony. This does not mean that tin separates or floats up from lead alloy that has tin in it.
Any time the alloy is above about 725 degrees, you will see tin oxidizing on the surface of the melt. It will still oxidize below that, but not so as to cause problems. A lead-tin-antimony alloy should not be above that temp to make good bullets. If you don't have a thermometer, you don't know the temp. The numbers on the dial on the lee pots simply means the percentage of time the heater element is on. 7 means 70% on, 30% off.