Sorry about this– figured it out, I wasn’t adding a !– comment — tag in the content field.
I just added 13 header elements to a website. 10 of them worked, three don’t. I did the same for all 13. I’ve checked and double-checked and I’ve done the same thing every time. The pages affected are very similar. To check, I created a simple text page but couldn’t get a header to display on it either.