While publishing to GMB, please make sure you do the following:

1. Choose the ideal aspect image ratio: 
Images and videos can helps for social engagement towards your business.

Google map allows you to add image in your list which represent the product to people that how it looks. They have certain practices regarded as an image ratio.

2. The GMB location must be verified.

It is necessary for your post to be successfully published on GMB that the location is verified. After completing your listing of your location, you must have to verify your business location in GMB and check the location's status by clicking on the Manage location.

3. Are you posting to a Google My Business listing that has more than 10 locations?

Sadly it is no possible, Google My business hasn't allowed its users to post more than 10 locations. If you added more than 10 locations then you'll receive a message from the GMB "This location belongs to a chain. The Local Post API is disabled for this location".

So far they haven't announced any changes regarding this. Once they do, we will let you know for sure.