Are the Amex transactions showing up in For Review but just not getting auto-categorized? If so, it’s because Enable suggested categorization toggle is off for that account.
In order to fix this:
Go to Bank Transactions.
Select For Review tab.
Click the Settings gear icon.
Scroll down and turn on Enable suggested categorization.
Once done, QuickBooks Online will help you review categories for automatically downloaded transactions and suggests categories based on how you’ve categorized similar transactions in the past.