How do I request someone I'm following on Twitter to follow me? I can't reply to them, so they must not be following me. Am I blocked from them?
If you are following the respective user, you are not blocked for him/her.
So, make sure is not one of these cases:
When you follow someone, they can be alerted via email that you have just become their newest follower. In their settings, there's a section on Activity notifications, one of which is:
When they're sent this email, it prompts them to check out your profile. They can either leave it at that or follow you back.
Busy and famous people probably won't have this set up as it creates too much noise in their inbox.
If you want to reply to someone, you can check out one of their tweets and click the Reply link. That will give you a pop-up with their Twitter handle prefilled. Enter your reply and if you're not blocked, the reply tweet will show up in their @Mentions tab.
If you're looking to Direct Message someone, then they would have to follow you before you can do so.
If they don't reply or follow you back, and you haven't been blocked, they don't want to reply or follow you back.