Thanks everyone for the replies.
I note the point that expression shows comprehension. However, her comprehension is apparently particularly good (according to standardised tests, which put her in the top 10%). It is her strongest skill.
Thanks also for the various points raised, and my answers may explain why I think shyness is the issue.
Littleturkish and TweedleDi
Yes, I read to her most evenings, and like to think I do "expression" pretty well :-)
shockinglybadteacher
I have asked, but she says she doesn't like reading out loud. When I have tried to explore the subject, she says she just doesn't like it. For want of a better expression, she is quite modest, but she is quite good at not disclosing information when she doesn't want to. Possibly she doesn't know, but I think also she doesn't want to discuss the point.
(also to littlejohndory)
Yes I do believe she comprehends pretty well what she reads. I sometime ask her by-the-way comprehension questions, and she gives answers that I think are fine. She picks up imagery and metaphor much better than I did at her age. What I do note is that I sometimes have to ask further questions to get the answers. I am very careful not to ask leading questions. It is as if she knows the answer, but doesn't really want to say what it is. She is the same with her maths.
pudcat
Don't laugh - but I have got her to pretend she is reading me a bedtime story. It helps a bit.
As an aside, some years back I had some kind of infection of my tongue, that meant in the evenings it was sometimes too painful for me to speak. She read her favourite bedtime story to me. (I should point out before anyone thinks I'm making her out to be a prodigy, she'd heard it so often she knew it word for word). At the time her expression was fine. It has got much worse since then.
daisychain01
We recently put her in an amateur dramatics class as we thought that might help. The tutor has come to us with the same complaint as her teachers. I am now worried that the classes might actually be counter-productive. While it was our intention that they help her gain confidence, I'm worried that it might actually damage her confidence more.
elvenbread and checkpointcharlie
We're in NZ - don't know what the curriculum here says about expression.
goblinlittleowl and WooWooOwl
My belief in her advanced reading standard is not based on my subjective assessment, but on her school reports.
Saucyjack
Yes, this is precisely my concern.
JustWonderingAbout
A good idea, but she won't sing (although she can). We can all sing. She used to when she was younger, but now she prefers not to.
enterthedragon
A very interesting question. When she speaks normally, she does speak with expression. However, it is almost as if she is consciously thinking about how to express herself. The voice seems almost put on... rather than just coming out naturally.
SDTGisAnEvilWolefGenius
Thanks very much for that suggestion - will try it.