Can ultrasounds miss breast cancer?

Yes. U/s is not a great screening test but an extension of the physical exam. It is very dependent upon the person (breast surg vs radiology tech?) doing the u/s, where they place the probe, how fast they move the probe, how much attention they pay to the screen, etc. U/s performed by a skilled breast surgeon can be very helpful and also used to guide sampling for definitive diagnosis and treatment.
Yes. Ultrasounds are best used as a directed test to further evaluate something that was found on a mammogram or physical examination. If a suspicious abnormality is seen on mammography and the ultrasound is normal, it is still necessary to perform a biopsy to check for cancer. Using an ultrasound as a screening test for cancer is not well established but may have a role for young women at high risk.
Yes. Ultrasounds and mammograms, though very helpful, are not perfect and on rare occasions can miss breast cancers.