No Network

Well there is no network on my airtel sim in united state at all. During my stay in hongkong i got network right way while plane was landing and got welcome msg from roaming service. But since landed in united state there is no network even not finding manualy...  samething happen my earlier trip in united states with airtel sim. Thanks god i brought two another idea and vodafone sims which are receivng network very well execpt...