Some of us have not been doing narrating from the beginning. Fortunately my children aren't too old to fix it! Some of my children find narrating easy and some find composing easy while others think they are getting their teeth pulled for one or the other. While they are doing it I feel like I'm getting mine pulled as well!
Older children can be taught why you want them to practice narrating and composing. You can list the benefits of it and they will quickly see why they need to do it. Younger children might not be so easily persuaded.
For a hesitant narrator I would have them start by having them tell back some of their favorite scripture stories. They can practice being descriptive and remembering facts. They'll be very familiar with the stories so they can be confidant in their material. I think that as they tell they will gain confidence.
You can then branch on to something physical that they are familiar with. "Tell me the habits of our dog Hank." My kids would find that hysterical and get quite animated about it.
After confidence as grown they may be ready to retell a chapter from a book you just read to them or they read themselves. Perhaps reading a simple book would help. Andreola mentions that some books don't have a lot going on in them and so they may be hard to narrate. She used On the Banks of Plum Creek as an example where there isn't much to say. However Farmer Boy is filled with lots of action.
I can see girls being more shy to narrate then boys. Some times some girls are not as self-confidant and perhaps feel that they may be doing something "wrong." Explaining that there is no right or wrong way to narrate so long as the facts are correct may help. Perhaps it may help them to only narrate with just you as the mother listening. They might not feel so sure of themselves if they have siblings listening in.
I haven't written much on composing yet. But let's pretend we all know what it is. : ) (It's just writing down the narration).
If you have a hesitant narrator than I would not move on to composing until narrating is going well.
Once the narrating is going well then mom writes down the child's narration. Then later the child begins writing down their own narrations. This is where a couple of my children throw up their hands and say, "no." These are my best narrators too! I THINK the reason is that they don't want to go through the manual work of actually writing. They aren't quick or real legible with their penmanship and this is where the breakdown is in our family. Also writing is physically harder and takes more work than talking.
My older son (age 10) is now seeing that maybe it would be good idea to work on better penmanship and as requested that he have a "cursive writing class." I think that as he gains confidence in his penmanship AND some muscle in his writing hand that he will develop lengthier compositions. In the meantime typed compositions could also be helpful.
So I have generalized because in our family there is a pattern. The girls (Liberty and I) can write for hours about something, but we aren't sure about orally telling you because "we might get it wrong and people are listening." The boys (including my husband and all the boys) can talk (hubby is a politician) and talk for hours. However they don't like to write because it "takes too long and it hurts their hand."
To conclude this poorly written essay:
1. Girls need confidence
2. Boys need hand muscles
Just some thoughts.