Purchase Order missing for some of the Sales order for few customers

Hi All,

There are few sales orders for which corresponding PO is missing.

But most of other Sales orders for the same set of customers, PO is coming.

Could you please enlighten me how this PO is assigned to the Sales order or it is updated with the Inbound Idoc ..

In any case what may be the reason for PO missing for these sales order...How I can explore this...?

Thanks,

Sanjeet