We've enabled Chatter in a sandbox but we do not see the option to actually create a new feed item in the UI, even as an Admin. I can create them via Apex so I don't believe this is a permissions issue. We can see feeds, like them, add comments, etc. Just cannot create them. Anything I might be missing? (We are employing the Profile Based Rollout)
You may also want to take a look at the "Global Publisher Layout" and check what Quick Actions you have in the Publisher Layout.
For example, if the "Post" action is included there and is the first one in the list, you should see the "Post" publisher in your Chatter page. You use that publisher to create a text feed item.