Hi Ramu,
Retention period must have been configured in your PI system. You can check this in NWA -> Message Retention.
For example: in our case the default retention period is 7 days. After 7 days the messages get deleted from PI system.
Maybe the messages got deleted form your PI system.
If this is your development environment, then you can resent the IDoc from your ECC system using transaction WE19 as follows:
Go to WE19 and enter the IDoc number for the PO in the "Existing IDoc" option and execute.
Then select button "Standard outbound processing"
A IDoc for the same PO will be created. Don't worry it will have any impact on your PO.
After that check whether that IDoc has reached the PI system or not and further to your supplier.
Important note: Please do this only in your non-production environment.
Regards,
Mateen