There are a number of possible reasons why the meeting rails on your double-hung window don’t align, prohibiting a sash lock from engaging properly. Here are a few common scenarios.
1. The upper sash has dropped (and has been painted shut in that position.)
Solution: An upper sash sash cord may be broken (or is too long and the weight is resting on the sill.) Repairing this will pull the upper sash back up to it’s original position. Or, the sash is hung-up somehow – perhaps on the head parting strip or weatherstripping. If it isn’t painted shut, it can be shoved back into position.
2. Zinc weather stripping isn’t seating into the kerf of the lower sash preventing the sash from completely closing.
Solution: Bend the weather stripping so it fits into the kerf. If the weather stripping is beyond repair, remove it and install in the sash kerf-mounted bulb-type weather stripping.
3. The upper meeting rail has warped, or the rails and stiles have separated, creating a gap which has caused the overall size of the sash to lengthen.
Solution: Remove the sash. Remove the glazing putty, glazing points, and glass from the rail. Remove any pins holding the meeting rail in place. If the meeting rail has warped, replace it. If it has just dropped, move it back to its original position and secure it. Then reglaze, repaint and reinstall the sash. Alternatively, trim a bit of wood from the top of the upper rail or the bottom of the lower rail.
A simple solution to all of these problems is to place a shim under the sash lock catch on the upper meeting rail. (See photo above.)