It turned out the issue was user error: we were having trouble getting a window to lock reliably, and the service tech showed us that because it's above a desk and you have to reach over the desk, it's easy to exert pressure in as well as down when you close it. You have to be very deliberate about pulling straight down on the window and not in. He showed us how the upper and lower parts of the window interlock in the middle, and how they won't interlock properly if the window is pulled in when you slide it down.
So, there was nothing wrong with the window. The service tech was completely understanding and nice, despite having driven a long way just to show us how to close a window.