We had a porcupine problem here at work a couple years ago and hired an ADC man to come in and trap them. He used salted apples for bait to catch them and placed the traps along walls because he said that porcupines have poor vision and will use things such as walls, chain link fence, etc. as a guide to get around and that they liked to follow the "edge" for guidence. SO maybe your sets are along an edge or on a well used trail that the porcupines use and need to get off the beaten path?