focusing with Canon EF 50 1.2L lens

I have a 7D and I often use the EF 50mm f / 1.2L USM. I have had a focusing problem with this lens during on location portrait sessions. I have been using the spot AF mode and selecting the closest point to the subjects eyes. There are times that I have to move a bit to compose the image. If I do this much at all its usually out of focus. I typically use off-camera flash so I can't shoot in bursts. I even have this problem if I'm at f/2.8. Any suggestions?