Please review the scenarios listed below and also review the attached video to correct your locking problem.
Push the upper sash all the way to the top while at the same time pushing the lower sash down. This will align the lock and will allow you to lock the window.
Tilt both sashes in as if you are cleaning the window. Push both sashes firmly back into the window channel. This should align the sashes and allow the window to lock properly
If the balance shoe becomes dis-engaged from the sash, the window will not be aligned and therefore not lock. To correct this situation, please refer to the “Window is not staying up – keeps falling down” section for further instruction.