Check these in order:

1. Auto-categorize Transactions (Client Config > Accounting) is No – nothing is categorized on import and the Re-categorize All button is hidden.
2. The transaction was imported by the scheduled background sync rather than the initial import – background sync stores transactions as uncategorized; run Re-categorize All.
3. The transaction is already reviewed – reviewed transactions are never re-categorized.
4. The description is empty, or no keyword is contained in it / no learned rule matches it exactly.
5. The rule's category is not valid for that transaction type – most expense and income categories can match either type (they just show as a refund), but Uncategorized Expense never matches a Deposit and Uncategorized Income never matches a Withdrawal, so a rule pointed at one of those two only ever fires on its own transaction type.
6. The rule was removed (removed rules stop matching immediately).

When a run finds no rule for an unreviewed transaction it is left in (or moved to) Uncategorized Income/Expense for you to review.