I've been including the HTML code from my Amazon Affiliate lookup at the end of posts that I make to my WordPress site. I switch from "Visual" to "HTML", enter the code (enclosed between <div> tags), and save it. But if I switch back to "Visual", make other edits, and then save it again, the Amazon <iframe> data between those <div> tags disappears. If I save it in HTML mode and don't mess with anything again, it will stick.
How can I make WP stop removing code I've specifically put in?