I set up an assignment rule on deals but it did not work. What might be the problem?

Assignment rules work when the following conditions are true:

1. Deal or contact is assigned to a group
2. Deal or contact is not assigned to a user (Assigned To field is empty)
3. Deal is not marked as Closed Won or Closed Lost

If you assign a deal or a contact manually to a user, then the assignment rule does not overwrite the value in the Assigned To field.