Possible causes in MSC side:
1. Prepaid sub’s SMSC address is different with postpaid and prepaid sub’s SMSC GT address is not defined in MSC
2. Is the SMS-MO service provided for your tested prepaid SIM? Are you sure about it’s profile in HLR for sending SMS?
BTW, check the issue in IN side and if you couldn’t find the cause, let me know the error which you are getting in A interface by getting trace or probe (If available)