The storage (or the FIT file storing the HR data and the steps data) is dammaged or full. Try deleting old activities from the folder //GARMIN/Activity, to see whether it helps, or contact the user OnlyTwo for the help with the FIT file repair.
The synchronisation does not work (the white circle around the watch symbol never closes) - in that case follow the tips in the Sync / Connection Troubleshooting Guide
When the synchronisation works (the white sync circle closes and turns green), but not all the data is appearing in your account, then it very probably means that your device is associated with multiple accounts, and a part of the data lands in the other account. It frequently happens for example when you reinstall the application, and inadvertently sign up a new account instead of signing into the original one. Follow the instructions from the support document Data is Missing from My Garmin Account After Syncingto resolve the issue.
