On the driver's page press Send test notification. The page says whether it got through, and if not, why: for example the driver has not allowed notifications in the app yet, or the app was removed from the phone.
Ask the driver to open the FleetKraft app, log in and allow notifications, then try again. On Android, also check that notifications for FleetKraft are on in the phone's settings and that battery saver is not restricting the app.