The most common reason this happens is because they have never been invited or have not accepted the invitation.

Invitations are very separate to users, an invitation is just that, a link to join, until that invitation is accepted and the user registers the user does not exist so cant re-set their password.

If a support ticket suggests the reset password is not being received check the user to see:

The user has been invited/is a user
The invitation is still pending

If the user does exist and it appears the email is just not getting to the user you can login-as that user and manually reset their password to something random from their profile page and email them that password.

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