You’ll only see posts if they’ve been assigned or shared (the To field on posts) with you or your company. If you have joined a project but are not seeing any posts, it might be one of a few different things.
Here are the most common:
You (or your company) may not have been assigned anything or added in the To field. If you should have been, just have whoever created the posts add you (or your company) and you’ll be good to go.
Someone from your company may have removed My company from their posts. This will prevent you (or others from your company) from seeing those specific posts. They can re-add this to individual posts or multiple posts.
You may have changed your quick filters - be sure it says Everything at the top of your Feed.