When using some symbology methods, larger symbols are incorrectly drawn on top; thus, obscuring smaller symbols beneath them.
Several symbology methods that should be drawing symbols in order of increasing size are not doing so. These are:
1. Pie chart symbology method where size is varied using the sum of field values.
2. Pies chart symbology method where size is varied using a field.
3. Quantity by category method with graduated symbols used to symbolize quantity.
To prevent chart symbols from being placed on top of each other, check the Prevent chart overlap box.