Please check that the top sash has got an even gap all around the way along the top and the bottom sash got an even gap along the bottom.

1. If you have bigger gaps at the end on either the top or bottom sashes, your head or sill could be bowed. This will stop your sashes not going as far as they should, and your window won’t lock.
2. If you have a gap at the corners, say right-hand side bottom and left-hand side top, your frame is fitted out of square, and this will stop your window locking.