Q: why wont my device back up to icloud

Make sure you have enough storage.

Manage your iCloud storage - Apple Support

Make sure iCloud Backup is turned on in your settings. Go to Settings - iCloud - Backup.

How to back up your devices using iCloud or iTunes - Apple Support

I also have this problem. I have plenty of storage and iCloud Backup is turned on.

What's my next troubleshooting step?

Here you'll find all possible instructions:

Get help backing up your device in iCloud - Apple Support

Actually it is two steps.

1. try another network

2. try to disable all backup options in "manage storage" and then when backup is successful enable them one by one.

If one of them is corrupted you will find out which one and if not you will backup step by step.