can you provide your document structures (input/output) how they look completely?
Did you try setting the InputArray/OutputArray on the LOOP-Step to the listTypes you are using?
These settings iterate over the list presenting only a document instance instead of a DocumentList.
On these instances you can call your service and maybe drop some intermediate variables before going to the next loop iterations. Maybe you need an additional MAP-step for this.
Deriving from your post your Documents are build like this:
item (list), put this one the InputArray of the LOOP