WhatsApp bundle not working

Every month I purchase the whatsapp bundle, and for a week now my bundle has not been working, unless I am in a WiFi zone or purchase airtime , then my whatsapp works, and I have never had this issue before, I have checked that my whatsapp bundle still has data left and it does. But every time I am not connected to WiFi or my airtime has run out my whatsapp does not work. I have never had this issue before and its becoming frustrating because its using the airtime and free data I get when buying airtime instead of using the whatsapp bundle