When you find that after syncing, part of the data is still missing in your Garmin Connect Account (for example the heart rate, steps, Intensity Minutes, or Activities), then there are basically the following possibilities. The most common and the most likely reason in most cases is the #3, despite that people are always persuaded they never created multiple accounts. Unfortunately it happens far too easily to tap the more prominent button Sign Up instead of Sign In, for example when reinstalling or restarting the GCM app.
- First of all, as also in case of most other problems with your device, the first thing to try, is a soft reset (typically - holding the Light button for some 20s, until the device shuts down), or powering down and powering up the device. It often fixes weird behavior, especially after firmware updates.
- Your Privacy Settings do not allow sending data to the Garmin Account (verify the option Storage & Processing), or the device is not set as the Preferred Activity Tracker (check out the Device Settings on the Garmin Connect Mobile app » Activity Tracking » Preferred Activity Tracker)
- The BT synchronization on the phone app does not work (the white circle around the watch symbol never closes) - in that case follow the tips in the Sync / Connection Troubleshooting Guide. Alternatively, you can sync the data with the GC account over the USB cable and Garmin Express on a computer, or manually by importing files from the device folder //GARMIN/ACTIVTY/ to the Import Data page of GC Web (in case of errors during the import, see the hint #4). On Android devices you can (and you should) also review the Device Sync Audit (in GCM Menu » Settings), and browse though the last audit reports looking for errors and warnings.
When the synchronization 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 (one of them may be a phantom account associated to the device without you being aware of it at all), and a part of the data lands in the other account. It happens very frequently and the existence of the phantom account is not always explainable. Follow carefully and in the described order all the instructions from the support document Data is Missing from My Garmin Account After Syncing to resolve the issue. When it does not help, perform the Master Reset of the watch/device - it will have similar effect, but you will need to redo some configuration and customization of the device.
The storage (or a FIT file storing the Activity details or HR data and the steps data) is damaged or full. Try backing up and deleting old files from the folder //GARMIN/Activity and //GARMIN/Monitor. Look up the last unsynced activities from the folder //GARMIN/Activity, and try importing them manually on the Import Data page of GC Web. If you get errors when trying to import them, try fixing the broken files by passing them through the Section Remover of FIT File Tools (without removing anything), and then importing again the repaired file(s).