hi nathan, try using something on the trap plate that the rats cant carry away - as someone else has said, nutella, peanut butter, jam something like that - they will have to physically stay on the plate to eat it and will more likely trigger the trap. If you are finding empty traps with the bait gone it is possible that mice are springing your trap but they are small enough to crawl out through the mesh.
You wont have to put a load of bait on, a dollop the size of a 50p piece will do - keep it in the centre of the plate so that the rats won't try and get at it from outside the trap otherw