You need to use the “Download old data” button in the Strava box in /settings.
Hi David.
I did exactly that for dates I was missing between January 5th and March 3rd 2023 but the date box will not let me go back further than December 22.
Have the same issue, starting from 1st of January and until start of May all activities are missing. Refresh on the “Fitness” page didn’t help
Thank you for an advice, that was the trick!