THE PROBLEM:
I was having the same issue last night. Spent hours researching solutions and I tried the solutions recommended by others in this post but to no avail. Finally I contacted LinkedIn about this issue and they responded right away. Their development team has implemented a new tool called "Post Inspector", which allows you to optimize content sharing. Literally, in just minutes this actually worked.
All you have to do is type in your URL and they do all the busy work i.e. verifying correct code of properties such as image, author, title, description, publication date etc. Not only do they verify, they also tell you what code to include, what is missing, and how to fix it.
Here is the website to use Post Inspector:
https://www.linkedin.com/post-inspector/