Hi!
When I saved the post and searched again, it appeared in the results list.
I suspect these posts are generated or imported from somewhere and not made via the post editor. A common issue with these is that the save_post hook is not executed after a post is generated.
Make sure that after the post is generated and all the data, tags etc.. are added, then this hook is also called for each post. This will tell WordPress to do the required maintenance on the post and execute the hooks attached to the save_post hook, such as updating the search index.