When the sash binds in the sash channel, the stop, parting strip, jamb or the sash may be warped or swollen. Furthermore a build up of paint in the channel can also cause binding. To break the bond between the sash and the rest of the frame, you will need to force a putty knife into the seam between the sash and the stop. Slide the putty knife in as far as it will go, ideally and the way to the side jamb. Repeat this all the way up each side of the sash. You also need to repeat this on the outside of the window between the parting strip and the sash. Now try and move the window. If it still won't budge, it may be necessary to remove the stops and then remove the sash.
After you get the sash to move or you have taken it out completely, remove any built up layers of paint from the sash channel. If the wood is warped or swollen, sand or plane the stop, parting strip or side jamb as needed to allow for proper movement of the sash. Remove a small amount of material at a time. Alternatively, you may be able to remove the stop and reposition it to allow the sash to move smoothly.
