I just realized this, but when you post on someone's wall, you can't delete your wall posts after someone else posted after you. I'm not sure if there's a set "being able to delete time" or what, and I'm not sure if this is supposed to be like this, but something I've noticed.
But if no one else has posted after you, you can delete your wall posts.
Just something I've noticed.
I know questions like this have been asked, but those were talking about posts from someone onw wall, whilst this is about you posting on someone else's wall.