I have a web service whose only exposed method instantiates a DLL.  The DLL's only job is to query a database to get a filepath to an image then return the image base64'd. I also have a test client to make requests to the web service.  It has a field in which the user enters an ID for the image and a submit button.   Everything works fine until we tried to stress test the web service.  Apparently, when using only two clients and starting the request at nearly the same time, the web service fails to return the images.  It sends back an error message stating that there is no image related to the ID sent. Is there any setting I need to change?  I've never run across this problem with a web service.  They're designed to serve multiple users, right?  So why is this happeng? Appreciate the help!

Software/Hardware used:
SQL Server, VB.NET