I have read negative reviews about the moisture control potting mix, it may keep the plant roots too wet. I've always grown the scented geraniums in full sun, which in my area is the sun received between the hours of 10am - 4pm. I'm not sure of your location conditions though - I'm up north. They should be able to handle dryness. For the amount of sun you're giving yours, it could be they are getting too much wet. The new growth on the bottom of the plant in your posted photo on gardenweb looks good and healthy, perhaps the yellowing leaves are just getting old? And your soil looks dry, when you water do you water until some drains out the bottom? Sorry I can't help you more.

I've had a couple of citronella plants for the last two/three years, but I may not be a good authority on them, because this year they're not doing well and I'm not sure why.
However, I don't think they like too much soon. Six hours definitely seems like a lot. Mine only get indirect sunlight (they're on my screened-in porch) for a few hours in the late afternoon/evening. They've done wonderfully up until this year. I believe they had a bought of scale, I think they're going to make it though.
Hope this helps, good luck!