What does it mean when our window screens are loose and the windows are hard to lock?
Sounds like too much insulation was stuffed in the jambs forcing the window jamb to bow inwards causing the sash to be to tight in the jamb. On the loose ones sounds like the jamb was nailed in with an outward bow. Use a 4' or 6' level to check and see if the window jambs are bowed from top to bottom.