Blog posts created in Shopify default to hidden when created, if you forget to change this to visible, Social Autopilot will ignore the post. It's also important to note that if you create a blog post and schedule it for a future date, more than 24 hours later, the app will not recognize it as "new" and not post it.
Social Autopilot goes by the post's creation time. For example, a post created on Monday and scheduled for Wednesday will not post.
Visit Blogs (links to Shopify) for more information on Blogs and how to create them.