Also, the ground cover surrounding and nearby the shelter can make a difference. Ideally, low and cut grass all around and away from anything else such as concrete, asphalt, gravel, wood, brick, etc.
That pretty much describes my ground cover--low-cut grass, and the shelter is not that close to any concrete, asphalt, gravel, wood, brick, or anything similar. It's also mounted about 5 feet off the ground.