Why does my iPhone randomly not download emails?

Sometimes [I have no control over or understanding of when] my iPhone decides it doesn’t want to download an email, so it shows up in my inbox, but the content of the email is blank, and at the bottom it says something like “download remaining [x]kb of this message,” and when I click on that, nothing happens. This is extremely frustrating. I think it started around when I upgraded to the new firmware after they released it on January 15. I use IMAP with Gmail.