Do you have positioning
lights in the front, above the window?
I have a similar problem and just before undoing the window I checked the positioning
light globes and the silicone around the
lights no longer prevented water from entering, and the buthyl tape inside was no longer not present enough to prevent water from seeping into the wall.
I cleaned, re-added butyl and resealed with a silicone gasket around each
light and my problem was fixed.