QUOTE(Clare1986 @ Nov 30 2008, 03:10 PM)

I'm not really sure how to describe it.... it's not particularly "bad" but it lacks depth or warmth. We're working on the Schumann Fantasy Pieces and I just feel that she's not achieving the sound that she we're perhaps both wanting from it - the same as when she was working on some Brahms last year. We talk about giving time to let the sound grow but although she's trying, I think it's her tone that is holding her back. It's a shame to see this frustrating her as she's such a good player! She's never tried any alternative mouthpieces with her usual teacher either - something that I hope to encourage while I'm teaching her.
I see - perhaps it's a little one-dimensional then.
A different mouthpiece may help, yes, but there are also other things you can try Clare.
Some long tone practice is invaluable. It is true that slow, beautiful pieces will also help, but to get an idea of exactly what individual notes are doing (or not!), you can't go far wrong with long tones.
Talk to your pupil about what sort of sound they're after. If they are, as you suggest, after something with more warmth, checking the breathing and airstream will help. Some people will say aim for a coldish airstream, some a warmish airstream - I'd say the best way to describe it is to think about it being not unlike breathing on a mirror to mist it up, but perhaps not quite as warm.
Certainly Clare, it's a lot easier to demonstrate than describe!

Check the airstream. Get your pupil to breathe on their hand the way they breathe down a clarinet, and tell them to describe what it feels like. You may well discover a lot from that.
Reeds that are too hard, and (particularly) too soft won't help either. Do they use one favourite reed over and over again? Guide them away from that practice if possible.
Do also make sure they're not biting, or squeezing the reed with their bottom lip to within an inch of its life - this won't help at all.
All the best Clare!