Read More logic is,
When the Post Style is Content in Appearance > Contango Options > Post Settings
You can use tag in the editor to execute Read More link
When the Post Style is Excerpt,
It will fire Read More link automatically PROVIDED the content length of the post meets the 55 characters at least.
So, i think the posts where you are unable to see the Read More link, have no or less content. So logically, there is no Read More content and no need for Read More link.
WordPress trims HTML markups in the except mode so you will unable to see the link in the excerpt format.
I have forward this question to the developer. They will fix this issue in the next update.
Thanks for your time and feedback.