A limited number of box sets have just been made available for users of the iOS app. Before you could only view on demand via TV Anywhere in browser (ie on your computer or laptop). There is a larger range in browser.
You'll need to register the device, and be either using the primary log-in credentials, or another login that was created from the primary account & granted TV Anywhere access. I'm assuming you're using iOS & have installed the TV Anywhere app?
Unfortunately, lots of people upgrading to the new TV Anywhere app for iOS are reporting problems with streaming (or lack of it) in recent days, so you might not be doing anything wrong. Not for the first time, Virgin Media aren't acknowledging the problem here, despite numerous similar or identical reports.