Arduino UNO capacitive sensor not working
As found on the Capacitative Sensor Library page (emphasis added):


  
  Use a 1 megohm resistor (or less maybe) for absolute touch to activate.
  With a 10 megohm resistor the sensor will start to respond 4-6 inches away.
  With a 40 megohm resistor the sensor will start to respond 12-24 inches away (dependent on the foil size). Common resistor sizes usually end at 10 megohm so you may have to solder four 10 megohm resistors end to end.
  One tradeoff with larger resistors is that the sensor's increased sensitivity means that it is slower. Also if the sensor is exposed metal, it is possible that the send pin will never be able to force a change in the receive (sensor) pin, and the sensor will timeout.
  Also experiment with small capacitors (100 pF - .01 uF) to ground, on the sense pin. They improve stability of the sensor.
  


The two emboldened points suggest that for a touch-piano, it would likely be advantageous to use a lower value resistor such as 1MΩ. Because the sensitivity increases as the resistor value increases, you want to use the lowest possible resistor value to reduce jitter &amp; interference among your signal wires.


Also, later on the page:


  The grounding of the Arduino board is very important in capacitive sensing. The board needs to have some connection to ground, even if this is not a low-impedance path such as a wire attached to a water pipe.


Ensure the Arduino is not run from a battery or unplugged laptop. For capacitative sensing to work, a stable ground connection is needed (via a DC plugpack or USB wall-wart is fine).

As a general note, try testing with only one capacitative connection first, then when that works, re-add the others.