My dd was similar in year 4 and it seems to have finally begun to come together in Year 5 - even I'm surprised at her improvement .
She had the ideas just couldn't get down more than a few lines often.Her teachers said her written work didn't reflect her ideas and ability.
I don't think keeping your dd in to finish work at break will work as an inspiration to write more though.I think it is very daunting to be faced by a blank sheet and start writing and check all the other things they are being told to do such as spelling,grammer,handwriting.
I did a search here last year and came up with a few ideas of things to do with her at home.Here are a few I can remember:
Tape record some of her ideas or you scribe for her.
Do consequence type games(verbally) where you create a story together taking it in time to add a line.
Look at a dull passage together and think of words you could change for exciting ones or you could add more detail to make it more interesting.
Pick a really interesting picture from a favourite book and write 3/4 sentances about what she can see.You can write alongside your dd , let her see how you think about which words to use and how you check your spellings and read it through to check it makes sense.
Mind map.Who,what,why,when,where gives a plan for a story.Spend 10 minutes discusing who was there,what they were doing,why were they doing it,when did it happen and where was it going.
If she is a good reader it should all be there is is getting her inspired to do something she finds difficult and giving her lots of praise to inspire her.Ask her teacher for what would help.