PM's still showing in your outbox means they have not been received by the recipient. Once they are, they will no longer show in your outbox.
So, either the member in question has not been online to see them, or has been online and didn't notice them. Some members unfortunately do not enable the notification of PM feature (either via popup or email) so they may sit unnoticed.
If you are urgently trying to get ahold of someone a staff member can retrieve their email address and privately contact them and request they login and check their PM's. We do not give out peoples email addresses publicly, but we can act as a go-between to get their attention.