Thanks Jack - good spot - that is it.
I have looked at installation instructions and yes they are oriented the right way. But this makes little sense to me why have that lower compartment and lip. When the slates are butted up close to the top lip, the gap fills up with debris and water can run straight in. I have cut back the slates with a grinder to make a channel and also used silicone to seal cap to slate but don't know how long it will last.
Anyone else used this piece of crap?