While SMS are leaving one SIM card, SMS are not leaving the other (same operator). You need to delete all messages on the SIM card. Proper communication with the GSM modem is visualized by the Tx A Rx LED on the appropriate port to which the modem is connected. If it connects to the modem at the start of RT (LEDs flash) and then no longer communicates with it - then this is probably the mentioned problem. Proper communication with the GSM modem takes place cyclically with a period of the order of seconds.