Why do only some friend statuses show up on Facebook?

This is due to Facebook now defaulting your Friends status updates to only show "Most Updates", instead of "All Updates".

You can change how much you want to see of a particular friends statuses in your news feed by doing the following:

1) Go to your friends profile page.
2) Then in the upper right corner, click on the "Subscribe" button.
3) In the pop-up, you can change How many updates you want to see in your news feed to the following:
- All Updates
- Most Updates
- Only Important
- Unsubscribe
4) Select "All Updates" to get all of their status updates to show up in your News feed.