I have had several "incomplete" mammograms.....the first time scared me to. As it turned out it was a shadow on the film that happened several times. I was even sent to a sugeron that said there was nothing to be concerned with. I don't know why or how the shadows appeared neither did the tech I could have moved slightly.
Will say a prayer for you that it is nothing and everything will be fine.
Take care and try not to worry.