sometimes some message does not show in notification. Is it normal?

Sometimes, some message from apps like wechat, whatsapp, facebook and many others. I am pretty sure that the notification setting is ON for those apps but the new message just does not appear in the locked screen, only if I click into the apps and find out that I've missed some new message.

Is it normal issue? Or is it because my device has problem? Or is it because of the possible virus or unstable iOS??

I am pretty sure that my network is fine because I am often connected to stable high speed wifi, so I don't think connection is the issue.

 

anyone???