I second the plate being the problem , the blade does not cut because its sharp , wouldnt be metal to metal if that was an issue , the blade cuts because it is thin.
What happens over time is there gets to be an indent in the bottom plate , you can either get a new one or as said above flip it over and enjoy the new chopper.
Hint, make sure you are on a solid base or right over the top of your bench leg , this will make sure all the energy is going to the chopper and you wont have to hit as hard, also do a light tap on one end and then the other, this seems to make it work better.
Dont hit it as hard and they last longer, some of the choppers I use here at the shop are 19 years old.