check that the hinges haven`t moved!
there is a recess on the frame and sash that the hinge fits into. often the width on the frame is bigger than the width of the hinge and with use over time the hinge can move to the outer edge of the recess. this is more common with top hung sashes. if this is so you will probably find the screw hole has become elongated and a new hole may be needed. the screws that hold the hinge are small self-tapping screws and a new hole can be made by using a powered driver going straight through the hinge and into the frame. make sure that the new hole will not interfere with the hinge operation before starting.
however the first thing i would look at is the locking mechanism, if it has cams on (small round lugs) that slide into a keep then these can be adjusted by use of an allen key or screwdriver. turning this will move it in a cam motion towards or away from the edge of the sash hence creating a tighter or looser fit when the sash (window) is closed.
tom, February 2011
