Another possible cause is a faulty overdrive solenoid. The overdrive solenoid is responsible for engaging the overdrive gear, and if it is faulty, it can prevent the overdrive from working. Replacing the overdrive solenoid can fix this problem.
Finally, a faulty overdrive relay could also be the cause. The overdrive relay is responsible for supplying power to the overdrive solenoid, and if it is faulty, it can prevent the overdrive from working. Replacing the overdrive relay can resolve this issue.