Why Can't I Find Someone I Unblocked On Facebook?
Three reasons: The person blocked you, after you blocked him/her. He/She may have deactivated the account. Something might be changed like user name, region etc which made the search engine of facebook to show that account atleast not in the first few.
When you unblock someone on Facebook do you have to Refriend them?
Unblocking them will not automatically add them as a friend again -- you will need to send them a separate friend request after you unblock them if you wish to be their friend again.
How to Unblock Someone on Facebook and Messenger [FAQ]
Why can't I add someone on Facebook after I unblocked them?
You can't automatically add them again. You have to start from scratch. If you mean add them as a friend then it could be that the one you blocked/unblocked has settings set so you can't (or anyone else). The settings for those who can send a friend request is only public and friends of friends.
I recently blocked someone and then unblocked them but now it won't let ...