Thanks for posting in the Apple Support Communities linopa,
When starting a SharePlay session, make sure you're following these steps:
"You can watch TV shows and movies during a FaceTime call with others. If everyone on the call has access to the video content (by subscription or free trial, for example), they can see the same moments at the same time, and use the shared playback controls to press Play or Pause. The show or movie volume adjusts automatically so you can keep talking while you watch.
* While on a FaceTime call on your Mac, start watching a movie or show in the Apple TV app 
.
* If this is the first time you’re using SharePlay, confirm that you want to use it. From then on, SharePlay starts automatically. If you select Start Only for Me, you are prompted the next time you want to use SharePlay for the Apple TV app. See Change SharePlay settings.
Everyone on the call who has access to the content can watch at the same time. People who don’t have access are prompted to get access (through a subscription, a transaction, or a free trial, if available).
While watching together, everyone can use playback controls on their respective Apple devices to play, pause, rewind, or fast-forward in real time. See Watch together using SharePlay."
As a way of isolating this, maybe test SharePlay by sharing music:
"You can get together and listen to music with others on a FaceTime call. If everyone on the call has the required access to the music, they can hear the song at the same time, see the name of the song and what’s up next, and use the shared controls to pause playback, reorder songs, add songs to the queue, and jump to the next track. The music volume adjusts automatically so you can keep talking while you listen.
1. While on a FaceTime call on your Mac, move the pointer over any song or album in the Music app 
, then click the Play button 
to start the music.
2. If this is the first time you’re using SharePlay, confirm that you want to use it. From then on, SharePlay starts automatically. If you select Start Only for Me, you are prompted the next time you want to use SharePlay for Music. See Change SharePlay settings.
3. Everyone on the call who has access to the content hears the music start playing at the same time. People who don’t have access are prompted to get access (through a subscription, a transaction, or a free trial, if available).
4. While listening together, anyone on the call can control the playback (pause the music, go to the next song, and more), manage the shared Playing Next queue, and view lyrics. See Listen to music together using SharePlay."
And that info is from Use SharePlay to watch and listen together in FaceTime on Mac - Apple Support
If you are still having issues with SharePlay, please contact Apple directly for further investigation: Get Support
Take care!
