Please try the following:
1. Ensure battery share is enabled by going to Settings> Battery> Battery Share> Allow Battery Share.
2. Check if your phone's battery is lower than the default limit in Settings (15%), in which case it will stop sharing power. To enable, go to Settings> Battery> Battery Share> Allow Battery Share. On this page, you will see the default settings is 'Stop sharing battery when phone reaches 15%'.
3. Remove your phone's case and other items that can cause interference, like metal objects.
4. Ensure both devices are in correct positions during battery share.
5. Check for device compatibility.
If the issue persists, contact the Nothing Support Team for further assistance.