If you're having trouble seeing someone's posts on Facebook, it may be because they've blocked you. If that's the case, their posts won't appear in your News Feed and you'll no longer be able to tag them or see any mutual friends you have. If this is happening to you, try reaching out to the person directly to ask why they've blocked you. It's also possible that the person hasn't actually blocked you but has just restricted their audience so that only certain people can see their posts. To do this, go to their profile and look for the audience selector tool at the bottom of their cover photo. If it says Public next to it, then anyone should be able to see their posts. But if it says Friends, Only Me, or anything else besides Public, then only people in those groups will be able to see what they post.