I'm also having issues with my forerunner 15 not syncing. If I plug it in, it will sync that days data, but it will not sync any other days prior to this and I can't seem to be able to update the data for previous days when it has not been plugged in.
When I check on Garmin connect it tells me that I have to sync my data but how?
It was working fine last week but has decided not to bother this week...
I contacted Garmin and customer service advised that this is a known issue right now with the Forerunner 15. It's not an issue with your device. It is an issue on Garmin's end and their engineers are working on it. Unfortunately, I was told that the only current solution is to sync on a daily basis.