Help needed urgent for checkout process

Hi Gurus,

      I have a problem regarding checkout using cXML. 

     We have a site where users login to the site by doing punchout to webmethods in response users gets back the url from webmethods which connects the  site and opens a shopping chart at there end, then they select the items from shopping chart and do the checkout when they do the checkout all the data goes back to the client system by displaying in there application. 

    Anyhow this process is running fine till yesterday for all clients, now we have a problem with a client. What they are saying that when they do the checkout then the e-catalog site gets hangs up but this working fine for other customers.

   I checked in the error logs of ariba adapter and IS logs as well but I haven’t found any error. Could you please advice what needs to be done at our end to find the cause or else what client has to do at there end.

Note: 1) Client is using the SAP system.
2) To sends back the data to the client system we are using the pub.flow.setResponse service.

Any help would be appreciated.