What is the required operating current in order for the sensor to be fully on and what is will be the voltage drop if there is sufficient current?
When the PLC's input is connected to 0V, it actually only supply about 5mA or less of current, which may not be sufficient to drive the two wire sensor. Thats why the voltage drop is not sufficient to turn ON the input logic.
Try using a 1K ohm resistor to pull up the common point where the sensor wire meet the PLC input. That way the sensor can draw more current via the 1K ohm resistor when it turns on and hopefully it will develop sufficient voltage drop to turn on the PLC input.
I hope this will work. If not, you may have to do a reverse logic, i.e. connect one end of a 2.2K resistor to the PLC input and the other end to 0V. This way the PLC input will be a logic '1' when the sensor is off. But when the sensor is ON, it conducts current and pull the input to above 10V which will then become a logic '0'.