I have been having the same thing happen on some of my lots in the last couple of months. I have been able to fix some of it but not all. So, what I figured out was that windows placed with MOO on don't drop most of the time. Sometimes, if curtains were placed on a window with MOO on, both the curtains and window don't drop. Windows on angled walls will also not drop. If you use the zero and 9 keys to adjust a window's height, it won't drop. And if you use Alt to adjust a window's position or the curtains on it, side to side, it also won't drop. The key thing I think, is MOO was on in all of my cases where the windows don't drop. So, try turning MOO off and placing your windows with it off to see if that helps you. Based on what I have experienced, that should fix some of it, but I don't think it will fix windows not dropping on angled walls. Those seem to do it irrespective of MOO being on or off.
Hope this helps.
