Inbound IDOC not getting generated

Dear Experts,

I am MM-functional consultant, while suporting one process I came across one issue in which an Outbound IDOC is created via PO output and this in return creates SO.

I understand that there is one outbound IDOC and it created another Inbound IDOC corresponding to earlier IDOC. Both IDOC can be seen as related IDOC's in BD87 if we check the Outbound IDOC.

In my case Outbound IDOC is procesed and is green but the corresponding Inbound IDOC is not generated. I can manually trigger the Inbound IDOC using T-Code WE19 and the SO is generated. But with this process I cannot see both IDOC as related one's in BD87 while checking the Outbound IDOC.

This process is working well in other environments/system.

Plese advise where should I look for cause.

Thanks

Manish.