@limalk is the PO Line connected to some project activity?

Beyond what Marcel already mentioned, another reason could be posting control. M92 if it’s non-inventory part (and typically then uses the purchase group as the driver). Are the values in the PO mapped in the posting controls and is pre-accounting for project enabled? 

If it’s a single PO that is not showing against the project, it’s typically the purchase group that is missing but if it’s all POs then it’s likely to be the pre-accounting posting controls.