There are two scenarios in which you may not be able to edit a post.
The first scenario is that the post was not scheduled, but published as a "Send Now." Therefore, it was only sent once and can not be edited as it is not scheduled for the future. If you'd like to resend the post, you can copy it and schedule it for a future time to be sent.
The second scenario where you cannot edit or delete a post is one in which another Gremln user created the post to the social network account that you both have access to. If another user created the post, only that user can edit or delete the post.