Either: 1. the object(s) you chose to populate on the ecosystem base object are too large 2.the presence of foreign objects is preventing the placement of your eco-objects if you checked "Decay Near Foreign Objects) 3. The overall density you selected is too small To fix this: 1. scale down the size of the objects you want to populate 2. uncheck the "Decay near foreign objects" box or lower the decay percentage value 3. Increase the density