Child pipeline is only processing the first document from Pipeline Execute

I’m using a sub pipeline to submit individual documents to a web service. This was working, but after some changes I am only seeing a single document processed to the service.

I have the re-use box unchecked, and pool size set to 1. I have tested each of the documents individually and they work fine independently.

The parent shows multiple executions:

But very consistently, I only see the first document processed end to end.

Any ideas on what can cause this?