Make sure that the magstripe of the credit card is facing the right direction. When you are looking at the side of the reader that says PayAnywhere.com, you should be looking at the magstripe as well. If you are using a case on your device, check to make sure the sleeve isn’t preventing the card reader from being plugged in all the way.
When the card reader is plugged in, make sure volume is turned all the way up. iOS only: Allow access to the microphone in the privacy settings of your device.