I put sand over the floor wire, and around the entrance to see what's turning up, by way of footprints some times it pays to fasten the door open and let them enter and take a bait a couple of times,to get used to it especially if it's a wary fox,but with the sand or soft soil you can see how close they become, to determine if there's a problem with the trap , some are trap shy remember this time a year theres plenty of feed around, they might not be pressured that hard till the cubs get bigger,if you can stick a trail camera up that will give you a good heads up as to what's happening,