Some tweets not appearing in my timeline, even though I'm following that person.

For certain people I'm following, I only see their stand-alone tweets, not their @replies to people (even to me). I have to go to their page to see their replies. And I do have it set to 'show all @replies'. Also, I have text updates set for that person and the same thing happens - I don't get any of their @replies on my phone anymore, but I used to get all of them when I first set it up. What's the cause of this? I tried unfollowing and refollowing the person and it's still happening.