Make sure you enter the correct passcode. It should be the one you made up for the Mobile App, not the one used on your phone - they are often confused.
Check your internet connection. If you suddenly lose your connection, the Mobile App may not be working properly.
Reset the passcode and create a new one. To do this, click "Sign out" below and log in with your email and password.
Can't log in to the Mobile App with a passcode
Updated over a week ago