The most common reason why your iCloud is not backing up your iPhone is that you’ve reached the iCloud’s 5GB free storage limit. As the capacity is full, iCloud is unable to import any more data from your iPhone.
So, now, what you can do is:
Pay for extra iCloud storage space. Learn more about prices in your region here.
Get yourself an iCloud alternative that has no storage limitation at all. Here you can back up your iPhone/iPad using iOS Data Backup & Restore.
Remove data from your iCloud storage. By default, iCloud will back up data from most of your iOS apps once the iCloud backup option is on. Here, you can disable some apps from iCloud backup settings so that your iCloud will remove the related data from that overfull 5GB space. This is how you do it: Turn your iPhone/iPad on, tap Settings > Apple ID > iCloud > Manage Storage. There, you can see a list of data that iCloud will back up. Disable those that you think are not important. Then, restart your device to refresh the iCloud backup record.