I've been having the same problem lately. The card is denied every now and then. Sometimes I try it again immediately at the same store and then it works.
But how weird is that? This isn't nornal functioning and they should do something about it. It's very unpleasant feeling going to the store with card you don't know whether it will be accepted or declined and you don't know the reasons for neither of that.
Have you reported that problem to customer service, have they replied anything and explained why is this happening?