I have niece who is 4 years old, and still doesn?t ...

If the medical doctors have ruled out an organic problem, take her to a child psychologist. This could be a developmental issue.

Additionally, if she has only been seen by a general doctor (i.e., pediatrician or ENT) then you should ask for a referral to a specialist so a complete workup can be done.

Once that has been done and no (medical) cause discovered, then you should definitely follow up on Elena's suggestion.

Also, do keep in mind that doctors are not infallible; it is entirely possible for them to make mistakes (including in diagnoses and/or lack thereof). The child's family knows her best. If they feel something is wrong, it is more than likely that they are correct (trust your instincts).

If they live near a large University with a medical link (UCSF, in my area) then they might try to get her seen and evaluated there; sometimes doctors in a University setting are more open to thinking 'outside the box,' as it were.

Good luck to you all.

Very good suggestions.  Your own doctor should have suggested a specialist.  It sounds like there is definitely some sort problem.  It is very good that you are being proactive about her care.  The sooner she is diagnosed and starts treatment, the better the prognosis (usually) is.    Easter Seals is a fabulous resource. You can contact them online, tell them your issues, and ask for a site close to you.  They have specialists and services available for those who need it.  Go to easterseals.com to get started.  You can look around the web site and contact them for further information.  Remember, the quicker the problem is diagnosed, the better it is for your daughter to get the help she needs.