I have that happen from time-to-time with my FR305. In my case, it's because my syncing clip is loose after years of use. What happens is my watch falls off the clip before it has a chance to sync properly. It takes multiple times to sync and eventually the GPS data gets corrupted. Now I use a clamp to keep my watch on the clip and as long as it syncs the first time, the data comes through.
A number of people encounter this problem for various reasons but often it's when the sync doesn't go through properly the first time.
