yup, thats true.It is easier to get great signals using two or more dishes than trying to get three sat locations with one dish.
dont know if the trees are any issue, but if you are having signal loss SOMETHING is an issue.
i know its snowing out there lately. snow buildup will cause signal loss if it accumulates on the dish. but if its simply raining or snowing, or just cloudy - you shouldnt be affected that much.