Some emails aren't triggering my Zap
Zapier uses a third-party service Mailgun to parse inbound emails. Mailgun has some limitations with the types of emails they can parse. For example, Mailgun will not accept and post messages without a present email address in the sender parameter. Some auto-notification emails (e.g., from Google Groups) will fail to be parsed by Mailgun because the sender's address is empty in the email.
My Zap isn't triggering on time
Email by Zapier uses a third-party service for inbound emails, so there is no guarantee over when the Zap will trigger and delays may occur. If you're working with time-sensitive emails, you can use IMAP by Zapier, or connect an email app that you have direct ownership of.