Does anyone have ideas on how to help a pre-schooler who is having difficulty with writing their letters?

I work in a pre-school cl in a public school system. I understand there are books and such to help students however, I am wondering if anyone has had success in helping a child write their letters. I have one student in the cl who has a hard time understanding the directions. She is a great kid and very quiet but when I ask her to sit with me and trace something, she scribbles. I'll help her by holding the pencil and will keep repeating but she still doesn't get it. I am not sure if this is age appropriate or not as 90% of the kids are on track with a few kids that have some trouble. The other day, I asked her to follow my pencil mark and she was doing OK but then she drew a line in the wrong direction and just kept scribbling. It's like she cannot comprehend how to trace or follow instruction. Any ideas would be great as I really want to help her and could use the knowledge with future kids. Thanks guys. I always like to look for people who like to think outside the box. Thanks.