Since you are an informed teacher/parent and you are at a loss here, I believe you should consider having her evaluated by a trained professional such as an educational psychologist. You could also ask your pediatrician for a referral to someone they think might help.
Sometimes things like this go away with time, but sometimes they do not. It will make you feel better to have someone who specializes in educational differences to evaluate her difficulty. They might have some suggestions for what to do to help your daughter.
Please keep in mind that our children are individuals, and they won't all learn at the same pace as others. Some walk sooner or later, some talk sooner or later, and the same goes for writing, drawing, reading, etc. Just when you think they'll never fall into step or catch on to something, they can seemingly master something overnight! It's too bad that schools have a cookie-cutter approach to kids, for the most part. It simply takes some children a bit longer to master certain things--perhaps because their little minds are already so busy learning so many other things! I know it's hard, but please have patience. It may take a lot of that, but the summit is near.