Having trouble hooking my S385-33 to the frame

So I just received the S385-33 and am watching the videos to install. The problem I am facing is that the hook on my window appears to be too high up on the window and I can't hook the channel into it and push it down far enough to get the top to catch in the metal stopper. There is no "give" in the string on the channel with the hook at the bottom, and there the spring is too tight for me to push it down. Not sure what I'm doing wrong...