When formatting all symbols, the buttons to modify symbol layers and symbol structure are missing from the Symbology format pane when the feature layer uses graduated colors with more than one class.
When more than one symbol is selected, you can only access the basic properties of the symbols. Addressing this limitation is under consideration for a future release.
See the first cell in the first table here: http://pro.arcgis.com/en/pro-app/help/mapping/symbols-and-styles/modify-symbols.htm