Hey @Tofu, 

Thanks for your post. The Samsung Health integration will only support syncing new activities.

At this moment in time, the only option to get historical activities from your Samsung Health account to Strava is by file export. If your activity has GPS data on it you can download a GPX of the activity using the following instructions:  

1. Go down to Exercise on Your Samsung Health app
2. Select "View history" 
3. Scroll down and select the missing activity in question
4. Once you are on that activity's overview page you will go to the 3 dots in the upper right corner and select "Export as GPX file".  Once the GPX file is downloaded you can then upload it manually to your account on the Strava website. 
 

Please note that since this is just a GPX file. The only data it will include are GPS points and timestamps, so if you had heart rate data on the activity in Samsung Health, that will not be included in the file, unfortunately.  You can manually upload your activity's GPX file here: strava.com/upload/select.