Activate the extended error report (with AT+CMEE=1) to get the error code and check the error cause in your AT commands manual.
Possible causes :
- SIM or SIM holder not properly inserted
- Lock of SIM connector is not locked (see SIM card lock feature)
- SIM failure
- defective SIM card reader, lSIM not recognized (5V SIM with a modem/module compatible with 3V SIMs only).
You can also activate the +WIND unsolicited by keying AT+WIND=1 for example. This command in this case of use gives the indications of a physical change on the SIM presence pin from connector meaning SIM inserted, SIM removed. Check the error code returned in the AT commands manual.