ajax form onSuccess not called

[John]

why is onSuccess not invoked but onValidate is?

    void onValidateFromBookingform() {
        log.debug("validate booking");
    }
        
    Object onSuccessFromBookingform() {
        log.debug("submit booking");
        booking = false;
        if (request.isXHR()) {
            return bookingZone;
        }
        return null;
    }