Whenever I give my chins a apple wood coin,(or any large "hard-to finish-in-one-day" chew toy) they'll chew on it for a night and then get bored with it. If I offer them a new one they'll take it and do the same routine, but if I offer them an older unfinished one they scoff at it... I'm sure this is normal, but why do they do this? There's a pile of unfinished wood blocks stacking up in their cage...I dont want it to go to waste! Anything I can do?