Hi.
I have an old, but yet powerfull, Forerunner 410. For some reason my activities are not triggering challenges anymore, and always work on the past. Actually some was calculating but sudenly all of them went to 0km and don't increase anymore. I tried quit and enter challenges again and tried to edit some activities to check if would count, but no success.
Any hints on what I have to do? (I don't use the app, only browser version)
Thanks.
Sure.
This is one example:
[https://connect.garmin.com/modern/activity/6214083162]
And if you take a look at my profile you will see a lot of other activities that should work with challenges.
You could try leaving the Challenge, and re-joining it. If it does not help, then enter all the activities that did not get counted, and edit & save them (just add a note, the effort, a calorie) or just…
... on the other hand, the activity is 7 hrs long, and there is a long ~4 hrs gap in the middle. Normally it should not matter, but I wonder where it comes from, and whether it is not a sign of some other…
All newer watches from Garmin now syncs with the mobile apps so there is not much need to use a computer now.
Exactly. And if you have no Android and no iOS phone, you could perhaps use an Android emulator on Linux - easier than using a Windows virtual machine. Assuming you'd have a device with BT support, and not one over a decade old 
