My old townhouse, the original builders would nail the prehung doors to the wall through the casing. Then they would just nail casing on the other side. Nailing jambs, nope since they were a hurry. So one fine day thirty years later as I'm redoing the casing to suit me, the door starts flopping around in the opening as the old casing goes away!
I'm with cork-guy. Call them up and have them fix it. Most home-building companies have a warranty department.