sliding window

When the weather warms up, I recommend sliding open both sashes and checking the master frame channel for any blockage that might be preventing the sashes from meeting correctly in the middle. Also, from the constant opening and closing the sash, the center part of the rail where the lock and keeper meet might be bowed and preventing the sashes to meet properly. You would remove the bowed sash from the window and use a rubber mallet to carefully hammer the side rail to the glass. You might have to unscrew the side rail and reset the rail to the glass if the bow won't pull in. I would recommend seeking the help of a professional if you aren't comfortable with this project.