It was the images! Thanks so much!!!!
I have clue re what might be going on here. I suspect that the events that have worked are ones that I have personally creatd, whereas the ones that don’t have been created by colleagues. I just created an event myself and it imported fine
I’ve just noticed that, after attempting to import these events and gettingthe 503 error message, they do appear in ‘Eventbrite Events’ section of the wordpress dashboard. Although the ‘Start date’ is blank, so perhaps the import is partially complete. They do not show up in the wordpress widget though.