Nikon d3100 video not autofocusing?

The lens are definitely on AUTO, and my settings are for face tracking. But when I'm taking a video, it doesn't track it all. I have to hold down the shutter for it to focus.

In the video mode, the auto-focus is much slower than when shooting still images.

Many people who use Nikon dSLR's to shoot video choose to manually focus their lenses

Those who use Canon dSLR's have to manually focus since Canon's do not have an auto-focus option when in the video mode