Frustration Over the Empirical Formula Lab

Why is it that every year we do this experiment and every year people get ratios that don't make sense? I thought that if you put the lid on your crucible right after the magnesium sparked (and starts to smoke) that you wouldn't lose material. This would result in accurate calculations for the ratio of magnesium to oxygen in magnesium oxide.
Right? Wrong. Every year people get the wrong ratio and I can't figure out why. Is it miscalibrated balances?
I have no idea. I wish I knew.