GSM Modem Interfacing Problem

hi frieds,
I am doing interfacing gsm modem with my com port through vc++ application.
we have three majore function
1) check signal strength
2) read sms using at+cmgl="ALL"
3) send sms using at+cmgs
it run perfect but after few days it creates problem in reading sms,
virtually it has sms in sim memory but when i check for command that time it shows "ok" response from modem .so it shows that there is no sms in sim memory
but when we restart modem than it start working fine
so please guide me about this problem.

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