I am also having the same issues. Been very long time but still no fix in place. I have raised a IT ticket for the issue - keep chasing it up but typical Virgin response is wait 10-15 working days and issue will be resolved. Many weeks/months later I still have the issue.
I find Virgin the most hardest company to ever deal with - absolutely shocking customer service!!!
Not sure if this is a fix but just found keep mobile data on during sleep for some reason it was off so when the screen is off you won't get notifications until you go into the specified app or establishes a data connection it should be on by default