Contact Garmin customer support and maybe the customer support of MyFitnessPal. I don't use it, but I have another app that does not import all of my activities in the past ~ half a year. I contacted the app and they told me that they are aware of the issue, and it started since a recent change Garmin made and they are working on it with Garmin. This was 3 month ago and it still doesn't work...
