Hi Angie - from my experience the ultrasound is used essentially to determine if a lump is fluid filled or solid. I would think that if they cannot find it on an ultrasound, it may just be because it is deep within your breast tissue.
A MRI, PET Scan , or Breast CT or CAT Scan are often other tools used in finding lumps. Sometimes the do one, sometimes they do ALL - which can be a pain, but at least it gives them a view from every angle and lets you know what to expect.
Good luck to you figuring out what that lump is!
Angi