A truism of automated assembly is that a robot is only as good as the fixture holding the parts. A robot can return to the same positions time and again. But, if the location and orientation of the parts deviate from what the robot has been taught, it may not perform reliably.
There are two ways around this problem. One is to increase the precision of the fixture-and accept a corresponding increase in cost and decrease in flexibility. The other is to give the robot a guiding hand with machine vision.