In your application the target object is very thin and it is not grounded too. So the parasitic capacitance it would add to the proximity sensor would be very less and that change in sensor capacitance may not be able to get significant difference in raw count.
For your application, you may need to have several thin traces connected at the end. In between those sensors you need to put traces of ground as shown in the image attached. Please note that the sensor traces should be thin and the gap between the sensor traces is equal to the metal ball thickness. The total parasitic capacitance of the sensor should not exceed 2--pF for better sensitivity.
Please go through the Manual tuning section PSoC 4 and 6 CapSense design guide.
Please note that you have tune the sensors in such a way that when the metal ball is placed on the sensor, the raw counts would go up by 5 times than the rawcounts caused by noise.
idea.bmp 4.1 MB