We had the same problem with a couple of our windows. I found out there is a tool you can purchase from your home improvement store to tighten the tension rod. You can find the rod inside the track of your window. Just tilt the window in as if your are cleaning your windows and you should see the rods on either side. You will need to disconnect the rod using the tool and twist it a couple of times the put it back in place.

Hope this helps,