Not all activities are uploaded to Strava - why ?

Hi,

I have connected my GC account and my Strava account in order to upload all my activities from GC to Strava.

It works well with all my Garmin devices, Fenix 8, Edge 840...

I also have a Concept2 indoor rowing ergometer, the logs of which import well into GC.

My question is: why do these indoor rowing activities not sync up to Strava automatically ?

Exporting manually from GC and importing to Strava works fine but is cumbersome :-)

Any idea?

Thanks