Sometimes the snake will not go for a previously frozen rodent because they are not of the proper temperature. They need to be about body temp, ie. 90F +.
If nothing else, hit the rodent with a little warm air from a hair dryer.
I also had to add that Becky gave some good advice here:
Also, there is nothing wrong with feeding on aspen or other substrates. Their stomach acids can break down bone, teeth, keratin, etc. A little cellulose isn't going to hurt a thing.