Before you buy a replacement board, try swapping the actual buttons from one side to the other. The buttons should have an internal spring and you might find that it's the buttons that feel different, rather than the opto interrupters.
Not all buttons are created equal - some definitely have a better feel than others.
If you're 100% sure that it's the plastic bit, then you can try gently heating the plastic and bending it back so it gives more resistance. When it cools, it should stay where you put it. Try this method at your own risk - I've done it a few times without dramas though.
