Today I made a sixth part independent of my assembly and added it to the assembly. Using top down the sixth part was edited in place to create a proper fit. On completing the top down, unprovoked by me, the fifth part and second inserted copy of the fifth part disappeared from the screen, just vanished. I opened the fifth part from the tree, it opened, was right and saved fine, but still no fifth parts in the assembly. From the tree the fifth part was hidden and unhidden to make it display again. The Solidworks factor in play. sw16sp4, K5000 with latest drivers