There is an easy way to tell if your messages have been read or not. If you have messages sitting in your 'outbox' it means that they have not been read by the recipient.
When a PM is sent to someone the forum uses the email address held against that member to send them an alert. However, if the email address is wrong they won't get this notification. Also, the notification might end up in their spam folder in which case they are unlikely to get the alert.
If they don't log in very often they won't see they have PM's waiting.
If that is not the case, they might just not want to reply
