As an FYI a really quick and easy way to find a post when you know the username of the person that made the post is to click on the username above the avatar.
Shown pic #1, located top-left.
After that, bottom right, click on “show all user’s posts.”
Shown Pic #2, located bottom-right.
Works slick when you know the username and the post is only a few days – week old, depending how much the person posts, of course.