Are you trying to use the gestures in a well-lit room? Better lighting allows the sensors to detect your motion better.
Edited to add: which specific gestures are you trying to do?
I am having the same problem. I seems to work in Gallery. But I can not get it to work with any browser or Microsoft Words.
Watching the various You Tube videos, they sure seem to be working fine. Eye gesture does not work either.
The gestures will only work with the stock browser as far as I know. When you try to use eye scrolling, are you getting the eye in the middle of the screen that starts out blue, then turns green when it recognizes your eye? Do you wear glasses? I could get eye scrolling to work on the S4 when I wore glasses, but others with glasses reported problems. Eye scrolling works better on the Note 3 for me.