Hi Amanda,
I have two gifted daughters, 7 and 10. The younger one skipped kindergarten, and has done very well overall. She did have a similar experience to your child in the writing area. Last year, in second grade, she could read miles above grade level, comprehend it, articulate it, but just couldn't seem to get it down on paper during her tests. Fortunately, she had a very understanding teacher who was willing to allow her to verbally answer her "story test" questions and the teacher would write them down. She only did this as time allowed, and only after DD "got stuck." The teacher told us that her ability to do the writing was similar to that of her own age group, and that she would likely catch up. Well, over the summer, the problem resolved, and she now proudly writes her own essay answers and receives very high scores.
Anyway, our experience showed us that just because a child is gifted, doesn't mean that all of his or her skills will develope at an equally high level. Hopefully, the issue will resolve with your own child as well.
Parenting gifted children is a challenge, isn't it? My friend Nancy, whose children had problems in school on the other end of the spectrum, told me one time that she always thought that people with smart kids had it so easy. She now realizes that this is not the case!
I wish you well,
lstrom