You are sending password reset links, but not receiving those emails, or you are trying to login with what you thought was the correct login without having success.
By default, the Marketo login is an email address. If you needed to reset password for a User, you enter the login which happens to be the email address associated.
However, there is a feature that separates the login and email address, where the login could be email@example.com, but the associated email address for this login is firstname.lastname@example.org. If this feature is enabled and you needed to reset a password, you would still enter the login (email@example.com) and the reset password email would be sent to the associated email address (firstname.lastname@example.org).
If you would like the feature that allows for separate login and email addresses, please contact Marketo Support.