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. In most cases, the Onename webapp detects such errors and re-processes the registration request automatically, but feel free to contact support and they can have a look at your case.
Sometimes the Onename webapp experiences heavy load or goes through a planned upgrade which can cause higher delay times than usual. Feel free to contact support to check about system status.
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.