My Vivoactive 3 will not syncronize with MapMyWalk anymore. Why not? No problems syncronizing with Strava

Sorry to hear that. Have you tried unlinking your Garmin Connect account and Third Party account and re-linking? If not, please refer to Garmin Connect Data Is Not Syncing Over to My Third-Party App and https://support.mapmyfitness.com/hc/articles/1500009118142-Garmin-Connect.

If you continue to have trouble syncing I recommend reaching out to Support. 

Hello, I have the same problem with my Garmin Forerunner 935. My activities don't sync with mapmyrun.

Yesterday, I have unlinked the two apps - which resulted in losing a few more activities. I then relinked the app  and all the missing activities getting synced.

Today, yet again my activity did not crossover from Garmin to mapmyrun. 

I have tried unlinking and re-linking - then it worked again …..and then it didn’t! 
I unlinked and re-linked once again - at the moment it works 

Mine stopped syncing with map my run  a week ago.  Disconnected map my fitness on garmin app and now it is not available to connect with.

Same here - and it not working anymore - very annoying