I've been having the same problem for months, I've contacted Samsung multiple times and keep getting told to completely reset my device, delete the cache and reinstall the app, but it didn't work and every time I did this it took a very long time and none of my settings or customisations had been saved. I suspected it was a problem with the API and pretty sure I suggested that as the cause but just kept getting told to do the same thing. I have the same experience of afternoon rides not uploading to Strava for many hours, but for the past couple of months I've found that other workouts (yoga) no longer sync at all. I want to record my heartrate during these sessions, see how many calories I burn and sync that (via Strava) to MyFitnessPal (syncing both Strava and S Health to MFP leads to duplication). I've resorted now to just using my old Garmin for bike rides, which doesn't collect HR data and requires me to upload via USB but it's more reliable, and here's the kicker - the HR data on my watch is completely random anyway, often showing 'flatlines' at the same HR for hours. For an expensive bit of kit that I'm tied into by contract for more than another year, it's really terrible.
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