Again, check the algae forum for the excel thread. it's a sticky.
What i was thinking about with the sun is that it goes across the sky, east to west, in an arc. During winter months, that arc is more southerly. Depending on someone's latitude, they might have a south (or north) wall that received zero sun for 6 months, and 100% sun for 6 months (during daylight hours, anyway).
This might have nothing to do with your issue. But it's a thought.