For new registration, the Onename webapp registers your username on Blockstack on your behalf and then transfers the ownership to you. It takes a while for all registration steps to get confirmed on the network. You will see a "Profile being processed" message on your profile until your registration is complete. The processing time is approximately 12-24 hours for new registrations (exact time depends on the underlying network conditions and load on the systems).
If there was some error in this process then your registration can be pending for a longer time. If your registration has been pending for more than 24 hours, you should contact support as it's likely that the registration was caught in our spam filters. Waiting for longer periods e.g., several days, is not going to help because we need to manually unflag your registration.
Some people try to register a lot of usernames through the Onename webapp and their registrations get flagged as spam. See this article for details on spam detection.