Same here. Just enabled syncing with Garmin on RunKeeper but they're not getting carried across. Not much of answer I'm afraid but would be good to hear if anyone out there knows how to get things working.
I've been (and still am for the moment) using Tapiriik for syncing my activities but I'm finding it a bit unreliable recently; hence looking at the direct Garmin RunKeeper sync. I'm guessing reliability issues are a common issue with activity syncing.