Because for some reason the images in them won’t load.
Where do you choose to view them? Because I know some platforms ask for your permission to load images in mails.
In my mail browser, on Firefox. Never got any prompts to apply permissions, though.
I don’t view my mails on a browser, sorry I won’t be much of a help 
Edit: Just so if you can’t find a solution, you can view level up email’s in this thread. Level-up emails
Just be careful to not give yourself any spoilers 
What mail provider do you use?