Not placed parts

I've build an assembly containing multiple parts in which I defined different instances using the family table. However 3 parts are missing in the model tree of the generic assembly, but  are shown in the respective instance assembly. I've turned on all model tree filters but that doesn't help. When I look in the Layer tree of the generic assembly there aren't any layers shown for the 3 parts. Used the find tool to search for the parts,they show up when as I not placed parts, as shown in the image. How can i replace the parts, or how to solve this problem?

Thanks in Advance,

Dave