I know that 80% of a guinea pig's diet should be hay. But my guinea pig Jack won't eat it! He has a rack full of lovely expensive Oxbow Timothy Hay and he is totally ignoring it! He also has hay for bedding which gets nibbled a little but not much. I have filled the hay rack maybe twice in the last 3 weeks, and he has only eaten any at all if he has run out of dried food. He seems healthy enough - should I be worried?