In your scene all faces are sharing the same UV coordinates, so just one item is placed to that position in the UV space.
Please, feel free to check attached scene with proposed solution. There is UVW Unwrap modifier applied with "packed UVs" to a rectangular grid and than Tiles map with corresponding number of tiles controlling the distribution for that Map Channel. This way each placed item is not sharing the position in UV space with any other item and it's calculated independently.
Hope that helps.
I must explain why I must use forest pro to scatter the objects on the conveyor belt now.
This is just the test scene. I won’t just place boxes on the conveyor belt. I will put some machine with Vertex Normal export from some CAD software such as Rhinoceros.
1. Railclone pro will be very limited in dealing with the objects with Vertex Normals.
2. There must be only one multi-material in one Railclone object. It’s not very easy to change the sample objects.
3. The Group objects is not accepted by Railclone pro as segment object.
4. There will be multiple layer of conveyor belt in my scene. It is not a simple conveyor belt on one plane.
The very important thing is Forest pro pack is good at rendering complex high poly objects with complex material. The Normal vertex won't be currupted too.
So, I must use Forest Pro Pack. Do you understand now ?