When you click the Show Full Post button, Discourse attempts to get the post content based on the topic’s embed URL. Generally this works, but it can fail due to the WordPress post’s markup. To fix this, you need to configure the Discourse embed whitelist selector setting. You can find details about how to do that in this topic: Configure the Allowed Embed Selectors Setting.
There is a fairly good chance that adding the following value to your site’s embed whitelist selector setting will fix the problem for you:
article .entry-content img, article .entry-content p, article .entry-content ul
You will need to test that. If it isn’t working, you will need to go through the steps given in the “Finding the CSS selectors to target” section of the topic that I linked to.