I was having the same problem trying to do about the same thing, What I found is Pin 20 and 21 are also wired to the top 2 pins (label SCL & SDA) next to GND and AREF and pin 13. Pins 20,21 are interrupt but the are also used for I2C.(In my case a I2C LCD) So it is easy to plug I2C in at the top and try to use Pins 20 and 21 as interrupts not realizing they are already being used. Hope this save someone a lot of time.