Lots of experience with these. There are a number of things that can make them leak.
1./ The seal. Open the window, clean the lens and the seal with a mild solvent cleaner (white spirit or PVC solvent cleaner). Smear the seal with silicone grease. Close. This may solve the leak. NEVER use Vaseline on the seals, as it makes the rubber go hard.
2./ Window gasket. The gaskets are foam. Over time the foam goes hard and brittle and starts to crumble. The only way to solve this is to remove each window and re-bed them using Butyl (Abromast BR) or a butyl tape.
3./ Lens deforming. Where the lens gets pulled up by the clamps, they deform, and can tend to let some water in. If this happens, you can try tapping the tangs up to make it close tighter, but you just end up making the problem worse. You eventually have to replace the lens I’m afraid.
I never open my windows (because every time I did, they started leaking). I eventually got fed up with it (and they were crazed), so got some new lenses made up. The new ones don’t open, but they don’t leak either.