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.