If your sync is stuck at 0 and you see a spinning wheel, this is typically caused by poor cellular reception. The poor connection to the internet doesn't allow all of your GPS data to upload.
You can try to sync your device again when you have better cellular reception. In some cases, we may be able to reprocess your session and push the data through using our admin tools. Email customer service at athletesupport@nznlabs.com or call them at (951)760-0917.