Have you tried those apple wood sticks? Mine wouldn't chew anything but they love to peel the bark off of those and chew them a little bit. Also I've been told to soak some safe wood sticks or popsicle sticks in some juice then let them dry and give them to the rats. Some rats are just difficult like that, lol.
Dont worry about it, rats dont need to chew to keep there teeth down, thats an internet myth. The brux to keep there teeth short and also eating hard food helps. If they enjoy it thats great but if not dont worry
I have a bunch of chew toys in the cage and only now am I starting to see chew marks on the wood blocks and vitamin chews. They had no problem, however, chewing a hole in their water bottle, my hoodie, and my blanket!
I just got a lava ledge 3 days or so ago, the girls are crazy for it and chew on it a lot! I think I will probably see more chewing as time goes on and I figure out what they like and what they don't care for