You can try soaking their chews in salt water for a day or two to make them more palatable. ChickiesnBunnies has soaked branches in mulberry juice. I have been considering doing something similar, but want to try boiling or juicing root vegetables (sweet potatoes, carrots, beets) and soaking the chews in the pot liquor or juice to see if they like it.
You can also give them other things to keep them entertained. Phone books are popular for chewing and shredding, as are paper towel or TP tubes. Stuffing them with hay is even more entertaining. My rabbits also like to play with tuna or cat food cans, and I also give them standard sized cans with both ends cut off. They are also handy stuffed with hay, but they enjoy rolling them around when they are empty too. Of course, make sure to crimp or file any sharp edges on the cans.