It could be a little snow on the dome, coupled with clouds, is enough to do it.
I don't have a dome, I have the Tailgater from Dish, and I rarely have problems. Only during heavy downpours, really. Can't recall any snow problems I've ever had, even with the snow sticking to the side of the enclosure, which I found surprising, since I expected to not get a signal.
I've read of people having to use some kind of repellent spray on the domes to deal with water on there, so the domes may be problematically sensitive, I dunno. Wish I had some advice for you.