Check your Strava settings for “My Apps” to make sure Zwift is stll connected. If it is and still not working then you may need to reconnect it:

https://support.strava.com/hc/en-us/articles/216918677-Zwift-and-Strava

There’s a bit about manual uploads, I think you’ll need to follow that procedure those for the missing activities if you want them to appear on Strava.