Hi Charlotte,
Unfortunately there isn’t a great deal more I can offer up here – it definitely sounds like the theme is the issue, yet it works perfectly for me when I test it.
So, it’s possible that this is either due to a specific setting with your theme or possibly an Events Calendar setting relating to your theme (have you played around with the Events > Settings > Template page and the default events template setting in particular?).
Please do report back if you discover the solution, however from our end I’m not sure there is much more we can do, unfortunately.