I am having the same issue, all my solder joints are good. I made sure the libraries are correct, and used the mifare classic format sketch. I noticed that in the youtube video that the pins SCL and SCA were not included in the soldering, I have tried both with and without those pins. I have also read the question of using SC2 or SPI come up often. Is there a way to select which method to use, or is it dictated by the version of Arduino that I have?The Shield that I have is version 1.0, not sure if that matters but I have seen a version 1.3 as well. when I plug in the power to the Arduino, the pwr and busy LED's come on and the the busy LED turns of for a moment, and then turns back on permanently.