Incorrect account connected: Although it seems simple, a common reason for people not being able to sync their activities with Strava is that they have paired a different/incorrect Strava account with Runna.
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For example, if you use one of the social login options (i.e. Facebook, Google) then Strava will create a new account for you instead of linking to your existing account.
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If you can see on the Runna app that your activity has synced to Strava, but you can't see it on your Strava account then this could be the cause. To resolve this, disconnect Strava from Runna on the 'Connected apps' page and ensure that you are signing back in with the account details that match your active Strava account's.
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Strava authentication issues: Occasionally, the mechanism that we use to sync your activities with Strava will be invalidated. This means that you will have to re-sync your Runna account with Strava. You can do this by going to your Runna Profile > Connected Apps & Watches page and toggling the Strava connection off and then back on again. After signing back in, you should be able to manually sync your desired activities to Strava using the 'Sync Run' button on the activity's page.