Just had 12 week ultrasound and they could not find the nasal bone.?

Technician had lots of trouble getting baby into a good position for photo. Doctor came in and only tried for a minute or so. All other markers were ok (neck skin, heart). I now have to wait more than a week for blood test results so they can give me a 1 in X prediction that my baby has down's syndrome. I asked if I could come back to have another ultrasound this week to try again to find the nasal bone but doctor said no. Should I go elsewhere to get one? Does anyone know how often they just miss the nasal bone and mistakenly increase your risk results? TIA