The Steptember app syncs data from your devices health app (Apple Health - IOS users and Google Health - Android users), not directly from your wearable.
To ensure your steps are tracked correctly, make sure your wearable is connected to and syncing with your health app first. Once your data appears in your health app, it will then sync to the Steptember app.
Please note - Connecting your wearable to your devices health app is managed outside of the Steptember app.