PO commitments

We have POs that did not create commitments in PA. 

We created POs using the standard PO interface, and then ran the PRC processes (PRC: Update project summary amounts).  Our understanding is this process would populate table pa_commitment_txns .  Some of the POs created commitments, but the majority did not.

We compared the POs that created commitments with POs that did not. (tables po_headers_all , .po_lines_all , po_line_locations_all l, and po_distributions_all) and nothing jumped out at us.

Can someone tell us what else we can check to see why some POs created commitments, and others did not?