Data recorded prior to the day you link your Garmin account to Motion Connected will not automatically sync to your Motion Connected account.

Motion Connected pulls data from Garmin every two (2) minutes.

The connection with a Garmin account/device works like this:

Your device offloads data to the Garmin app or software.
The offload point holds the data until it uploads it to the Garmin servers.
Garmin notifies Motion Connected that your account has new data.
At the next 2 minute increment Motion Connected pulls the data from your Garmin account.

Network issues or large amounts of data can cause delays in between these steps.  For example if Garmin gets data from your device at 10:03 and Motion Connected doesn't get the notification that there's new data until 10:09, the next pull may not happen until 10:11 and due to a large number of accounts being updated it may take until 10:15 for your account to be updated and show your data.