WhatsApp Messenger not showing "last seen" for a contact
There's a relatively new privacy feature on WhatsApp that can disable a person's last seen. When disabled you can see when the person is online but can't see when they were online last. For the person enabling this setting, it blocks others' last seen automatically as well.

You can read here for more information on privacy settings:
https://www.whatsapp.com/faq/en/android/23225461