I assume she's still within the height/weight limits for the seat?
I had a Roundabout, but my son grew out of it at about age 2 or so. If I remember correctly, it didn't have a slot to move the buckle forward to give a little more room in the leg area. Maybe the straps are too tight? Maybe they need loosened a bit? I know they are supposed to be snug, and you've probably already tried that, but I thought it worth a mention.
As for the new one you bought, even if it's out of the box, you should still be able to return it. I've returned several carseats on my quest to find just the right one, and had them all out of the boxes, and in my car. Still had no problems returning them. Worth a try.