Indeed, as authors cannot edit posts made by anyone else. This is how the system functions. An author can only edit and delete his own posts.
If you want help with a few of your posts, once in a while, you can consider giving the author in question authorship of those few posts.
To do so, Edit your page, Scroll down until you see the Authoring information field set and click on it to open. Under Authored by type the name of the author. The field includes an auto-complete feature, so entering a few letters should be enough to get you a list of possible authors, click on the name that represents the author. Then save the changes.
Later, once the author is done doing his work on that post, you can change the Authored by name back to yours. It protects the post from further editing by the author and shows your name as the author of the post which is probably what you want.
Authors can only edit posts that use the Filtered HTML or Advanced HTML input filters. If your post is set to Full HTML, then the author will still not be able to edit the post (even if he's made the author of that post.) And remember that changing from Full HTML to another filter results in loss of colors, font sizes, font names, and some other things.
If you want an author to help you with many of your posts, then he needs to be made an editor. This is done by editing your author account and selecting the Editor checkbox under Roles.
Your website settings let editors edit all posts on your website.
Although they cannot delete someone else post, they can unpublish a post1, which is nearly the same as deleting since the content will not be visible by anyone but you and editors.