The token "input" is returned when validation fails, and the Action class is never invoked. If the validation messages are not displaying, the most likely cause is that a property is being validated that is not present on the page (so there is no where to display the message).
If the validator is being attached to the Action class, note that a subclass will inherit the validations of its superclass.
Try submitting the form in debugging mode to see if there are any orphan field validation errors.
Type conversion errors may also cause a return to "input".