Sometimes a XML fail and a document is not created in the SRM. Checking in the SXMB_MONI the XMl is in error and is not possible restart it if the root cause is not solved. Checking the error message and doing a research about it can help, but sometimes it is not enough. Even in the /N/SAPPO/PPO2 when the document is in error it is not possible process it if the root cause is not solved.
In this case the better option is debug the initial process to understand why the XML is failing.
The SAP KBA 2222756 provides a step by step how to start analyzing of a XML in error. It contains some useful Classes, FM that can be used to figure out what happening with the document, also it has a PDF attached providing a step by Step how to start the debug.
The hint helps in all scenarios with ESOA, per example, with Purchase Requisition creation, Purchase Order and its Confirmation, SUS documents, etc.