Thanks Idolse
The code I posted above is displayed here in the Kindle Previewer. The first h1 of the file is displayed. It displays on a new page in the ePub but not on any of the Kindle Device previews.
I read your tutorial yesterday in order to solve the issue. That's how I found the xPath I posted. Only when it didn't work I decided to ask for help.
I don't know why it would work 10 times and break on the 11th.