Check if you have a prominent image on your home page, normally the output code for the RRSS takes into account the highlighted image of the page or post.
Once I put the highlighted image, I would recommend you go to the facebook URL debug ( link ) where you can enter your URL get all the data that facebook sees to check if the image is seen.
In the event that this problem arises with the articles / posts, I would recommend that you install, if you still do not have it, the Yoast SEO plugin ( link ) since in that same in addition to touching for SEO it leaves us the option of RRSS where we can put a description and image for both facebook and twitter.
I hope it helps you!