From your attachment, please goto settings and remove the "Disable private messages" selection and also Enable Private message notification.
Or just follow the selections, from the attachment i sent you.
Then ask your friends to test it.
Requesting all forum users to use your fingers to type some meaningfull TITLE in each thread you start, instead of being lazy, and using EMOJI as title, Add a proper title name. If you can't add propper title to any thread you start, do not ever start any thread.