Sorry, my reply to you earlier looks to have been lost, let me try again.
Your theme PHP template for single posts can be overwritten by any of Elementor Pro templates, Views Content Templates, and Layouts templates, and although this normally has a predictable hierarchy where Elementor templates override Layouts templates which override Content Templates, it could be that your Elementor template is being overridden if you have more than one kind of template assigned to the same post.
If you edit one of the posts in question you can see if there is a Layout or Content Template assigned (I don't know how to do the same in Elementor). If you want to use the Elementor template then you shouldn't have either a Layout or Content Template assigned.
In fact, there is no reason to use two page builders on the same site, and you should almost certainly disable Layouts altogether.
Next, edit your Elementor Template and make sure it includes a text module that outputs any-old text for testing, and verify if it is shown on the front end.
So you can determine whether or not the template is being used, before addressing whether it outputs what you want it to.
