PartsCurrent not reading an assembly


I am trying to model a system where part of an assembly is unloaded, “worked” on and then returned to a different node where the rest of the assembly will be waiting. I thought I could set this up was by using the GetProperty and PartsCurrent but it appears that PartsCurrent doesn’t read an assembly as a part.

At the line_30 process the worked on part is re assembled to the “cart”(White rectangle) and then sent to the Done process to be reassembled to the main assembly.

Essentially I’m looking for a way to only send the worked on sub-assembly when the rest of the main assembly is present. And when their is no sub-assembly ready the main assembly continues on.

Any advice or help you could offer would be greatly appreciated.
Thanks in advance!