If the tracks and droppings are that of a fox, then, okay you have a digger fox. If he's been there 4 or 5 times then at least you know he's coming back. At that point you should be able to determine where the fox is standing while digging up your trap. Conceal another trap where he stands to dig, and brush it in to try and blend it in so it looks just as it was the way he left it while digging up the origional trap.
If he digs that one up also, go back 18 inches or so from the second trap and bed in another trap just as you did the second one. you should be able to grab him by the back foot.