When this problem occurs the green light on the card swipe will not blink when the card is pulled through the reader. It is most prevalent when the reader is mounted to the top or the right of the display. It only seems to happen with the Magtek Sure Swipe readers (p/n 21040145).
The problem is associated with RF noise in the environment, some of which comes from the PS150 display. Grounding the PS150 Card Swipe Mounting Bracket resolves the problem by providing an RF sink to isolate the card reader.
See attached document for details.