Hi Saif,
Try this:
1) Login to Configuration and Monitoring Home of your system. Go to Adapter Engine --> Message Monitor --> Database tab and then select the message that is failing.
2) Click on Open Message button and then you will see different versions of the message processes through the system.
3) In the last step you should be able to see the JSON message going out of the SAP PI system.
Note: you may need to check what level of logging is enabled on the system as well.
Regards,
Sanjeev.