Make sure that your card reader is fully charged and that a green light is shown when plugged int the device. You may need to remove the protective case on your device if you can't get the reader to fit securely in the headphone jack any other way. Make sure the slots are clean and free of lint and debris.
Make sure that the card's magnetic stripe or chip is facing the proper direction. Chip cards need to be inserted into the top black part of the reader with the chip facing toward the gray card image. When swiping cards without a chip, use a slow and even motion along the length of the slot located between the gray and black sections of the reader. The magnetic stripe should face the black portion of the reader.
If your reader still isn't working, give us a call at 408-295-8360 so we can help troubleshoot.