Even after unlocking mobile not working

hey hi everybody

I have a NOKIA 6200 (Mlife AT&T USA) which was earlier locked. After unlocking the phone i tried using my SIM , but the signal would never come . Of the 4 Cellular service providers in the area , it finds only one and the rest it does'nt find (My SIM is not of the company whose signal it finds). I thought maybe the phone is not unlocked , so i tried putting the code again , but it gives the message "phone was not locked". SO what does it mean?? Are its other frequences still locked or what?? why does'nt it allow the frequency of my network/service provider?

Thanks
Harry