Rear window motor slides but window doesn't open

I bought my 2012 RTL a few weeks ago, but foolishly didn't test out the rear sliding window at the time. Now with the Ohio weather finally starting to break, I'd like to get some airflow going while still being able to hold a conversation, but the rear slider won't open. I can hear the motor moving and there's a definitive sound when it gets to the end of its track, but the glass stays in place. What do I need to do to fix this, so I can open the back and enjoy the spring air?