Drilling holes to hang curtain rods - HELP!

So I am trying to hand curtain rods. I’m drilling holes above the window to mount the brackets, but when drilling some of the holes my drill stops about a quarter of the way through. It’s hitting something really hard and won’t drill through. Does anyone have any idea what it’s hitting or how I should proceed? (The curtain rod is going above a window facing out of the front of my town house. It’s on the main level. The house was built in 2016).