Check the specification of the remote end RS232 (peripheral). RS232 comes in flavours... The original RS232 standard asks for a Voltage swing between +15 V and -15V. but used to work fine with +/- 12V. The chip (MAX3232) on the RS232 converter however provides + and - 5.5V max which is generated using charge pumps.... these pumps have limited power (drive) capabilities. All be it that this works fine for most devices it would not work for devices which accept only signals in line with the original specification.
Also, do not use long flat cables to interconnect (>1m). The relative high capacitive load of these cables will have a negative impact on the integrity of the electrical signals.
Boosting the signal level for TX sometimes resolves the issues and is not hard. You need a dual power supply of +/-12 V i.e. 24V between the "0" and "1" state. 2 switching transistors and a few resistors would "boost" the signal. Note: with additional components, a distorted signal (long cables etc.) may be overcome too. do not omit the reference ground...
You typically only need boosting the TX (transmit from the converter to the receive of the remote device), the transmit of the (old skool rs232) peripheral typically provides more than enough power to the input of the Max3232/arduino.