I was watching NowTV through the app earlier and it was fine with no buffering. It's probably a temporary issue with NowTV at their end.
I'd run some speed tests because there's nothing wrong with the app because that works fine. It's could be something to do with your set-up or network congestion in your area.
The only time I have had an issue with NowTV via the Youview box was due to a national NowTV fault
Try updating the app and software on the box. We've been using it for a few months without an issue so it's not a specific problem with the App. If it was, people would be talking about it here and also on the NowTV forum and Doendector would be flagging up issues as well
I have found that the NowTV app can be very sensitive to bandwidth availability changes especially with the HD Boost. That mainly happened when the went 1080p but I haven't experienced it since they launched on BT though. Other times I found that it can start buffering during a film streaming at 1080p and then I have to start the stream again. Personally, I just think the app on all formats isn't particularly efficient with data and memory management but on the flip side, the 1080p picture quality makes Netflix's equivalent look a bit poor