Wooden windows can swell in wet weather. Try rubbing candle wax along the sticking edge of the window. If that does not work, plane away some wood. You can plane the opening edge of a casement. Take off enough extra wood to allow for painting.

Unscrew the casement from its hinges so that you can plane the sticking edge.
Tap a screwdriver with a hammer to free painted-in screws.

Movement of wood due to damp weather can also make fasteners difficult to close, or lead to loose, rattling windows. Fix the problem by adjusting the positions, as if you were installing from scratch. If adjustment only requires minimal movement, you may have to move the fastener so you can attach into solid wood.