My simple freebie for email address sequence has failed.

Hi! I signed up for Pro trial & used the mailing list template ( freebie for email address) Has worked for some people using the trigger word and not others. How can I see which users the automation has failed on, why this happened and how to prevent this in future please? maybe i did something wrong? If it's not fixable,I won't continue with the pro account as my main aim is to grow my mailing list.  Thanks.