hello masud,
have you checked interference problems ? perhaps your hardware is fine, but there are bad radio conditions (bad quality, poor coverage or high interference)
Also, can you check on which TRX is located your SDCCH timeslot(s) ?
Maybe this TRX does not carry any call (calls are allocated on other TRX), that’s why there is no call drop on this TRX, only SDCCH drops.)