In general, there are three ways to diagnose a failure in communication.
First, look at the log files of both OPC applications. Often they will indicate if the application failed to receive a request, or failed to receive a reply. The log files will also indicate whether or not the application sent a request for a reply. This will help you to isolate the cause of the problem and determine whether the source of the miscommunication was on the OPC Client or Server end. Note that the amount of information in a log file is controlled by that application’s vendor. OPC specifications do not specify the type of information to log. Nevertheless, logging any information that would help a person find the cause of problem is good practice.
Second, look at the Windows event log to find out if there were any errors logged by Windows. This will also help you to isolate the cause of the problem.
Third, use a third-party application to log the OPC communication between the OPC Client and OPC Server. These applications (often called "sniffers" or "loggers") simply capture the communication between the OPC Client and server and record all the calls in a log file. The integrator can then read through the file and determine the cause of the problem.