While marrying the Neopixel Matrix to a spark.io Sheild Shield, I couldn't seem to get the matrix to respond.
Through testing with other Arduinos (worked), I tried it un-married and with the data pin coming from digital pin 2 instead of the hardwired digital pin 6 -- and it worked.
I don't know if I have a bum Shield Shield or if there's something in its hardware that doesn't play well with pin 6. In the end, I physically trimmed the #6 header pin off the matrix and wired pin 2 to pin 6.
Details of the project that lead to this issue are on johnkeefe.net