The most common reason is that you have not installed the latest stamper application for the new Loopy Loyalty application.

If you used the old Loopy Loyalty application, that old stamper app does not work with the stamp cards created using the new Loopy Loyalty application.

Please make sure you have installed the latest stamper app on your device:  

For more information on using the new stamper app please visit: http://docs.loopyloyalty.com/issuing-stamps-and-redeeming-rewards/how-to-issue-stamps-with-the-loopy-loyalty-stamper-app