Sometimes it is best to have the pull cord running down to a treadle plate on the floor as a door trip mechanism. They are easy to modify with a bit of wire mesh. Failing that, extend the cord so the bait actually lays on the floor slightly.
No self respecting fox expects to see a rabbit hanging 2ft up at the back of a cage. As with all trapping, try to make everything as natural as you possibly can.....Good Luck.
Rolfe.