SoapUI not signing a service request

I have a SoapUI project pointing to a vendor's SOAP service. I've attached the proper keystores and set up the authentication properly. There are many operations in this project and every one I've tried works, except for one. All point to the same URL, and have the exact same header. However, looking at the raw output from the service, it is clear that all but one of the operations is being signed. All the operations are in a single WSDL and I don't see anything special in the individual specs. What could be wrong with this single operation?