if you have a big enclosure then things could be spawning inside. but mobs won't spawn within 30 meters of things like workbenches and forges, I forget the full list but even a campfire will block spawning and it doesn't even have to be lite. I've had it happen but once I put some campfires in the corners and along the walls things stopped spawning in my place.

Another posibility is that its just weather damage. Only things immune are Stone, thatched roofs and the darkwood items that use tar in their crafting. But from what I've heard weather damage will never drop an item below 50% so if the items aren't getting destroyed odds are its just exposure. Might not be the reason but it's something to look into.

Good luck.