Sometimes the cervical os is so tight that it is not possible to pass the biopsy device. Often the placement of a tenaculum on the cervix, to stablize the position of the cervix and straighten the axis of the uterus (and os), can be helpful. If this does not allow the passage of the sampling device, then attempting to pass the uterine sound, while applying traction on the tenaculum, will allow enough dilation of the os that the biopsy device can be passed.
If you are unable to pass the sound, then you could wait until the patient is menstruating (which naturally increases the size of the os) and attempt EMB again. If waiting seems inadvisable, then the patient must be referred to one of the Other Methods for Evaluating AUB used to establish diagnosis.