The PC and Mac versions of BT Cloud should continue to back up your files from the backup point last reached whenever your computer is switched on and there is an internet connection. On your mobile devices, backup should continue whenever the app is open and there is an internet connection.
The most common reasons for backup failures are a lack of network connection or not enough free storage space.
To check how much free storage space you have click on the "Cloud" icon at the bottom of your screen. This will open up a box which will show how much space you have left.
If you are on a PC or Mac, check you are connected to the internet. If you're using wireless, try improving your signal by moving closer to your BT Hub.
If you are using BT Cloud on an Android device, iPhone or iPad, check your network settings to see if you have an internet connection. By default, BT Cloud is set to only back up over a wireless connection. This is to help prevent you incurring excessive mobile data charges. However you can change this by going to Settings and turning the "Wi-fi only" setting off.
You can also open the ‘Automatic Backup’ section in Settings to check that automatic backup is turned on and all the expected files are selected.