Missing a square clip from the corner top of lower window.

Thanks for posting. It looks like you're missing a Top Sash Guide. These are nylon parts that keep the channel balances secure to the window sash. It's a bit odd that the window still works for you. Typically when one of the guides break, the balance shoots past the sash and lodges itself near the top of the window jamb. The video below shows how to repair a jammed window like this, and how to install new Top Sash Guides. NuPrime windows often use either 18-110 or 18-110B, though there are exceptions, and there's no way to determine which you need without removing and comparing. Let us know what you find.

Alternate view of 18-110

Alternate view of 18-110B