Hi,
Are you in Java or .NET ?. A method invocation data collector (MIDC) could help.
Do you have access to the WS client code ?. If yes, you're on the right path, look at the code and locate the service method and its parameters.
Then create a new MIDC, describe the class, method and parameters in the dialog (Configuration/Data Collectors). Don´t forget to target the MIDC to new transactions and existing ones. Save your changes, and wait for new snapshots.
You could also start a new diagnostics session on the BT(s) that use the method, to verify your setup is right besides the thresholds of slow/very slow.
Hope it helps, regards,
Luis