Tweets not showing up in my timeline (but showing up on person's profile)

I'm experiencing an issue where not all tweets from the people I follow are showing up in my timeline. If I follow X person, then I'm only seeing about 20% of their tweets in my timeline - the rest simply don't show up. If I go to their profile/page and look at their list of tweets, they show up, but they don't show up in my timeline. I've tried both Twitter on Chrome, the Twitter iOS app and the Tweetbot iOS app and it's happening across all of them. I've tried both the "top tweets first"...

Tweets not showing up in my timeline (but showing up on person's profile)

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