I don’t know if this is addressed directly, but I found through trial and error that spawning props with larger defined radius first is important. I had also set my spawner up to spawn plants and grass, which were much smaller and more plentiful than the larger props. When I was initially spawning in the small plants first, they were leaving much less consistently valid space for the large props to try to spawn into. Switching order fixed this and the smaller props had no problems finding room to squeeze in around the larger guys. 
If anyone wants ideas for other ways to spawn items, this general problem is known as the Knapsack Problem
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