Please try to disconnect and reconnect Garmin from your account settings in Grokker. Are you logged into the Grokker app all the time on your phone? If so, please try to relaunch the app and then try to sync Garmin again. Please note below how Garmin works with Grokker and the activities we record:

From the time you connect Garmin to Grokker, Garmin will only sync back up to 48 hours, but will then sync your workouts going forward.
We record activities tracked by devices only. Manually entered activities are not used.
We only record the following Garmin Activities: walk, run, bike, swim, hike, elliptical, and workout.
We do not sync any steps.

If you continue to have issues, please email support@grokker.com