Modem hangs

hello everyone,

                 Im using Q2686 wireless CPU for designing tracking kit, when modem enters from a high signal strength GSM level to a low level GSM modem gets hanging. Again in my code im restarting the modem for registering SIM to GSM network. So can anybody explain why this hanging of modem happens when it enters into different GSM signal strength.

By modem hanging you mean it does not re-register to the network???