One cause of the Worldpay/Vantiv credit card reader not working is that the triPOS service isn't running. To fix you can do the following

  1. Press Windows Key + R
  2. Type services.msc
  3. Find TriPOS.NET (hint: start typing tri to skip down to it in the services.msc window)
  4. If it says 'Start the service' click start otherwise try clicking restart

Wait for the credit card reader to restart then test. Something else you can do is open notepad from the windows search and paste the following.

net stop TriPosService
net start TriPosService

You can download a file to restart the credit card reader here.

Then you save it as something like 'Restart Credit Card Reader.bat' (without quotes). Save it to the desktop then whenever you need to restart the chip reader just right click and run as an administrator.