Hello Lore M,
I don't really know how the Goe My Wordpress works, but probably it isn't a theme problem. Maybe you should test other mapping plugins for Wordpress which you can find e.g on https://nearplace.com/blog/best-wordpress-google-maps-plugins-free-paid/. If everything will work with the other plugins you should also check if you are generating your API key in a good way, you can compare your methods with the methods from our tutorial https://nearplace.com/blog/how-to-add-multiple-markers-on-google-maps/ and then if everything works correctly you probably should contact the support of the plugin.
Cheers, Amy from Nearplcae