Just go to Configuration workspace> System Settings> Password Settings> Select the Restrict Password Validity box to set a limit on the length of time a password is valid.
When you select the box, two fields become active: In the Validity field, specify the number of days a password is valid. In the Warning in Advance field, specify the number days before expiration that you want to warn users that their password will expire.
Please do not forget to assign points for guidance.
I've never experienced this problem before but others have mentioned it in the past here in the forums. Do a search on password and you'll see what I'm talking about.
If the notification that comes with the application doesn't fulfill your expectations, you can always think of creative ways to work around it.
The rep_accounts table holds the password modification date value for each application account. You could extract this information from the database and create your own script to notify users via e-mail or use web-API...