Hi andyfromhatfield,
We understand contacts you add to the Favorites tab in the phone app aren't showing. We're happy to help.
If you haven't already, we'd suggest restarting the device to begin and see if that corrects the issue.
Restart your iPhone
Do you have your contacts stored directly to the device, or do you have them synced from an account like iCloud or a third party email address? If you're not sure you can check in Settings > Mail > Accounts, then tap on any accounts you have set up to see if the Contacts option is turned on.
If you do have your contacts synced to the device from an account, try turning the sync off, wait a few seconds then turn it back on. After doing this, go back to the phone app and try adding a contact to the Favorites tab. Then see if it is showing up or still missing on that tab.
We hope this helps. Cheers!