I do sympathise with your predicament. I have had in the past amazing children who could achieve greatness, and yet they wouldn’t. In time I learned detachment.
I think it will be difficult to do anything unless you know what exactly is going on. Perhaps she just need a break. Does she actually enjoy playing? Or does she do it because she is being “forced” to (there are many subtle ways of forcing children to do things)? Whose idea was it in the first place to learn the piano?
If she enjoys playing, perhaps you should just give her a couple of months with no specific assignment, just let her play for the pleasure of it.
You may well be right in both your guesses (things have become difficult – perhaps rather than just hearing praise now she has to do with criticism as well and she may not used to that – or it may be peer pressure).
A great motivator is to watch other children of the same age playing. Last month there was a concert on the local school which several of my students attended. The headmaster asked the children who played an instrument and would like to take part on the event. A number of my students wanted to do it. Others were shy about it. In any case, on the day of the concert, there were several children (aged between 5 and 9) who played the violin, the cello, the piano (some were my students), the recorder (again some were my students) and the school choir (which just started this term). The audience (parents and relatives) was very warm and enthusiastic, the children were very relaxed and played beautifully. On the days after the concert, my students were on a practice high! (both the ones who took part and the ones who did not – they now want to be in the next one, so they are practising hard to be up to it). It was as if something had clicked and they had understood the point of it all. They finally got the idea that music is not some boring activity that you do in isolation who knows why. There is a point to it, and the point is to share it. (Besides, being the centre of attention and getting all that applause helps!). Anyway, is there any recital, music festival, school concert you could take your student to? It might rekindle her interest.
Without knowing exactly what the cause is, this is all I can think for the moment.
(And keep in mind what liiw has said above. He is quite right).
Good luck!
Best wishes,
Bernhard.