Question about Cyrus CS45 XA

I've got this phone, all' perfectly, buy still remain a Problem... The whatsapp notifications... frequently i can't recive them. Suddently I recive the message only when i open the app. In the past I had other Cyrus phone buy With android 7 or 6 and in tose phones the problem didn't appear. Is It normal? I've controlled the notification manager, but all still appear correct...