Backup not working even though devices are connected to power, wifi and locked. Is there a way to manually force backup?

That depends largely on:

a) what backup system you're using

b) why it's 'not working'

Are you backing up to iCloud? or to a desktop system?

If using iCloud, you can go to Settings -> AppleID -> iCloud -> Backup and click the 'Backup now' button to manually initiate a backup.

The commonest cause for iCloud backup failures is lack of space in your iCloud Drive account. Have you verified you have space there? (Settings -> AppleID -> iCloud -> Manage Storage)

You have the ability to create a backup manually to iCloud from Settings. You can also take a look at these troubleshooting steps for the inability to create or complete an iCloud backup. If you can't back up to iCloud or if restore from iCloud backup failed - Apple Support