There could be many reasons why you can't see your friend's posts on Facebook. One reason could be that somehow that person has been removed from your friends list. Another reason is that your wall is set to sort by top stories. Click the word Sort under where you type a status, and select most recent. Also, make sure your subscribed to your friends status updates. Visit their page, hold your mouse over the word Friends, and then make sure "Show in newsfeed" is selected.