There is a very good diagnostics script provided by oracle support it self.
When you execute the sql script, it produces the html output,
which shows all the required data to troubleshoot specific PO (document/ ECX Transactions issue).
XML Gateway Data Collection Script for Outbound ECX Transactions (Doc ID 742618.1)
Key areas to check in diagnostics output.
1. Check whether the OTA is running or not.
2. Ensure that Protocol Type and Protocol address is correct
3. Check the Exception Text in ECX_OXTA_LOGMSG part for Errors / Error Message, to troubleshoot the issue further.
-- Narasimha Rao