Your post is probably not ranking high enough for the search term containing just the post title. I believe it still shows up, just not on the first page.
When you added your domain to the search, it made your page more relevant for that search (which contained your domain name) so you ranked high. But searching the title alone without the domain increases the number of other potentially relevant pages competing with yours for that term (which does not contain your domain name).
Hope that makes sense.
If it's a new post it may take some time or require some more SEO work. And this is highly dependent on factors like competition, backlink profile etc
Also try searching the post title enclosed in quotation marks e.g "Post Title"
