Hi,
Seems like I got the anser. A missing part occurs when the current order quantity in PO can't fulfill current requirement.
For example, current requirment from reservation is 100 pce, and current stock is 9 pce. You create 2 POs seperatly for two vendors, one with qty 11 pce, another 80 pce. In total the current stock and order qty can fulfill requirement. But when the first delivery comes and you post GR with qty 11 pce or 80 pce, at that time the material will be a missing part. By the next GR it won't be.
These two POs in below screen shot will triger missing part pop up screen in MIGO.
Br, Joey