The most common reason that a domain may not transfer in a timely manner is outdated contact information. Network Solutions, the most commonly-used registration service, sends out a confirmation email to the administrative and technical contacts for a domain before acting on any change requests for that domain. If confirmation is not received from one of those addresses within three days, the request is dropped.
If the email address(es) of the contact(s) are no longer valid, it is not possible to transfer the domain via email, and you will need to contact the InterNIC directly to effect the change.
Other reasons include: the InterNIC's processing queue is backed up or one of the contacts did not allow the transfer to occur.