Hello,
Is this for wired or wireless? What vendor and model switch or wireless controller are you using? Is this occurring for all End Systems or just some of them? Make sure the policy and/or vlan that is assigned after registration occurs allows access to the NAC interface so that the HTTP redirect data makes it to the End System. You can get a trace from the switch port or AP to see if the redirection packet is making it to the End System.
Regards,
Scott Keene