Making sure the mouse\rat is rather warm via soaking in hot water prior to feeding helps. Some times people use a small piece of soup bullion in the soak water to add some aroma,this is to entice the snake. Then there is a method that you take a pin\needle and spear it through the head of the rodent then move it about as to scramble some of the heads insides,this is another way to make the mouse more appealing but is a little bit too far for most people to go. You can also try waiting a week longer than your usual feeding and see if its ready to eat then due to hunger.And then sometimes it just takes time,I have seen some snakes refuse for moonths then one day they decide they will accept a frozen food stuff.