Sensor conveyor doesn't stop the part at the right position

Hi all,

I am facing a problem when using a sensor conveyor, please see the screenshot below
the red box is the item and below it is the sensor conveyor with yellow stripe location where the box is supposed to stop.
Initially when the 1st box arrives it stops exactly at the middle of the box (exactly at 3rd compartment)
but when the second box arrives it gets little ahead of the yellow sensor line ( see the screenshot) but it is supposed to exactly at the middle
Please help me with this problem