Dear, yuli!
Your problem is located on VLR-HLR-interface and you better to get trace from it. As you said, sometimes only 1 request is required for succesfull registration, sometimes 10 or more in single roaming network. Hence, most likely than not there is routeing problem on signalling path between VLR and HLR, especially if SIGTRAN (IP-network) is used for transportation of MAP-signalling between VLR and HLR. Also congestion is possible in your home network or in transit network.