Hi @Anonymous User
What model smartphone are you using?
Have you tried deleting the NOW App and download a fresh version from your phone Apps Store?
After downloading the App, maybe temporary power down the phone before signing back into App.
Are you using the same login details on your phone Now App has when checking the Mobile Membership is showing on your online account?
I am also assuming you are attempting to watch one of the five permitted channels on a Mobile Membership which is Premier league, Action, Arena, Racing (Horses) or News.