There's no known issues we are aware of with regards to Whats App but if you believe the Child Safe option is potentially blocking this as some other community users have suggested then you can turn this off via My Virgin Media.
Try going to your virgin media on line account, go to your settings and turn off your child restrictions. No idea why WhatsApp can’t be used when on but this worked for us having tried everything else including reinstalling app, resetting WiFi, new media box etc👍
I tested this out myself, see post here, and it is specifically the Chat option in the categories list that needs to be de-selected. This means that you don't have to totally disable Child Safe/Web Safe.
Access My Virgin Media
Go to My Apps > Web Safe
Scroll down to Categories/Websites/Set timer below Child Safe and, on the Categories list ensure Chat is not enabled