iPad not receive FaceTime calls when asleep

My 6th generation Ipad WiFi, model MR7G2X/A, using ISO 13.3, has over the last 6 -9 mths stopped receiving FaceTime calls when asleep. Also it does not show any missed calls.. when I open. I have reset the IPad, checked and rechecked all settings, reload the FaceTime app, changed modems, check all online advice, all without success. FaceTime receives and make calls without any problems when the IPad is awake.