Hi RossMac13,
Welcome to Apple Support Communities! We appreciate you reaching out for help with this issue and the Nike Training Club app. We'll be sure to get you pointed in the right direction.
Unpairing your Watch and then setting it up as a new device was certainly a thorough troubleshooting step. If the following steps help to isolate the issue to the app itself, we may refer you to Nike for direct support.
First, let's see if your Apple Watch and iPhone are otherwise working as expected. You'll want to ensure that both devices are fully updated to the latest software version. Which version of watchOS and iOS have you got installed? This can be found under Settings > General > About on each.
Have you had an opportunity to restart both your Apple Watch and iPhone since noticing this behavior? If not please do so now.
Here's a guide with the steps to restart your iPhone:
Restart your iPhone
And the following guide is for Apple Watch:
How to restart your Apple Watch
If you find that the issue persists once each has been restarted, we can recommend deleting the Nike Training Club app before reinstalling it, and testing again.
Have a great day!