If you double-click your layer in the table of contents and select the Symbology tab, under "Show:" you have Features, Categories, Quantities, Charts and Multiple Attributes. This is the normal list... But, for example, if you select Categories -> Unique Values -> and click Add All Values -> Ok and then go back to reopen the Symbology tab, the whole Categories section goes missing. Please see the attachments. The first attachment shows the normal list and the second attachment shows the missing Categories section. Thanks.