Hi Alejandro,
Okay, so this happens when users are releasing outside of Fiori and Workflow. But not always, I assume.
How does the workflow handle this case, does it listen out for the event that is created when a PO is released? It could be a problem with when that wait-for-event step is created. if it's created too late then it may miss the event. Look at the workflow design in SWDD.
regards
Rick