subheadings disappearing

In my most recent notification email that went out to my subscribers, the email that shows the entire article in the email has empty spaces where the <h3> subheadings should be. The <h2> subheadings display just fine, but not the <h3>. I know that it is supposed to display them because the Styles tab allows us to format <h3> tags. Nevertheless, where the <h3> subheadings are supposed to be, there is simply an empty space between the preceding and following paragraphs. Thankfully, the subheadings were not totally critical to the article (it wasn’t a long one), but I’m sure this could be quite problematic for some, so I hope this can be resolved quickly. Thank you!

Note that this was both in the editor and the emails sent.