Not sure how you made your line, but try using the "Generate > Beam" effect. It easily allows you to control the end points of a line. You could use some simple expression to attach them to your boxes.
Johnny Cuevas, Editor
"I have not failed 700 times. I have succeeded in proving that those 700 ways will not work. When I have eliminated the ways that will not work, I will find the way that will work."
---THOMAS EDISON on inventing the light bulb.