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If a person'a record hard bounces, check if anything in your system is marking them email invalid. If it is, Marketo will not send an email to that person. If a person's email address is updated, and their record is still marked as email invalid then they will not receive the email. You can set up a smart campaign that is triggered off of email address is changed, flow step put them on a list and then you can inspect the list manually and reset their email invalid to false.
Just check to see if those folks that did not receive the email the last time were marked as email invalid.