PO BPM failing to call service reference occasionally

Hi PO BPM Gurus,

We have multiple PO BPM’s running at the same time in our environment, each BPM is doing a WS call to SAP ECC to post transactions.  During the day, some messages will error out in BPM saying that cannot invoke the service reference without any other detail.  In this case I just restart the BPM and the goes in cleanly.   What could be the possible cause of error?

Rgds,

Yves