Channel Balance broke.

I replaced my window balances with S385-18 for Caldwell 17 2 00 I measured them as 18" They went into one window ok the Hook for the pullie system was a little bigger but we managed to get them in (the springs are bigger than the ones that were in there). One of the balances popped the string and broke when we pulled down the window all the way. I don't think we did anything wrong but thought I would check. I plan on ordering another one to replace the one that broke. Thank you. Joni