I had that happen to me once a couple years ago -- the capsule sat in my stomach the entire time and never moved over the 6-8 hour recording time. I had been feeling pretty nauseous the week before the procedure and even landed in the hospital, so I guess my stomach just wasn't empyting into the small intestine like it should. Probably some small bowel inflammation was backing up my whole GI tract.
I re-did the procedure a few weeks later when I was feeling better and it went through my stomach in 10 minutes!