Some people have difficulty enoying 3-D for reasons that can be physical or neurological. My best friend cannot fathom depth in movies at all. Active shutter (or even passive) glasses might not decode the same amount of depth for one person that they do in the next. It's rather like being more sensitive to (or not sensitive at all to) certain colors in the spectrum, i.e. color-blind. I believe many people are 3-D blind.