The most common problems that block a domain from being transfered are thw following:
The domain currently has the Domain Lock feature enabled. This is a security feature across virtually all registrars that prohibits certain changes to a domain. If a domain is locked, transfers and DNS modification cannot be made until the domain is unlocked.
Attempting to transfer a domain to a new registrar within 60 days of the domain being renewed or transfered. The central registry will not allow a domain to be transfered to a another registrar if the domain was registered or transfered within the last 60 days.