Unnecessary chewing + no chewing at all

[Felixle]

My hamster, Nala, is just over a year old, in that time she has gone through about 8 cages, 6 of which she chewed her way out of. She chews on her cage constantly. Her cages have all been over the recommended size and the current one is double. She has always had things to do, wheels, things to climb, tunnels, but all she does is chew. I don't often see her teeth, but I did the other day and they are stumps, that doesn't worry me. I just don't understand why she's chewing, to me it seems she just likes to chew on metal mesh or plastic, she's not interested in wood. She doesn't need to chew all night when her teeth are barely there to chew with, I just don't get it. On the other extreme my other hamster pringle who is two, has never really chewed in his life and has really long teeth and I don't know what to do about that either, so any help for that would be great too.