At 3d shows, i notice the 3d effects maybe 2-3% of the time. the rest of the time, the picture looks the same with one or both eyes open. all the tests i've tried suggest stereoscopic vision/depth perception is fine ... so why can't i see 3d?

Apparently about 10%: of people can not see in 3D. Because humans have both eyes in front, the possibility of having them work together to create depth perception exists. The fusion in our brains of two slightly different pictures from each of our "cameras"/eyes gives the sense of three-dimensionality. Lazy eye and strabismus are conditions that interfere with eye coordination. It can be corrected, esp in children.