An important lesson for new fedi users ( 
) :
If it looks like an account blocked you because you can't see their posts but you can't think of a reason they blocked you (e.g. you've never interacted with them, or if you have it was cordial): it may be that the admins of their instance has defederated with yours.
That's not blocking you personally; it's an administrative decision that usually comes from conflict or poor moderation by the other user's instance. Instances that tolerate abusive behaviour from users might be defederated because their actions conflict with the rules and principles of the admins of other instances. Admins can block specific users as well, but defederating is often a wiser choice. There are a ton of absolutely toxic/hateful instances created to harass people, and if you haven't seen them that's because they've been rightfully blocked this way.
A lot of different things can lead to this that the individual users aren't aware of or responsible for. It could be an admin behaving like an asshole. It could be questionable admin choices related to security. It could be that the instance is harbouring abusive users. This could happen in either direction—the other user's instance might have been blocked by your instance's admins, or your instance might be the one higher blocked by theirs. It's worth looking into. Some instances list the ones they've defederated in a section on their main page, and there may be a reason listed as well.
If you find that there are some accounts you can't follow because of this, you may want to consider migrating your account to another instance. Sign up for another one and see if you can follow the people you currently can't. If you can, then you can consider migrating your account. This will move your following and follower lists (but not your posts—get an archive of those) so you don't have to rebuild from scratch.
This is a thing that happens, and it's part of the design of the fediverse to give smaller instances more control. It's not personal.
