If you are using .Net client then you may not have set
//This says how many outgoing connection you can make to a single endpoint. Default Value is 2
System.Net.ServicePointManager.DefaultConnectionLimit = 200;
here is the original question and answer WCF Service Throttling
Update:
This config goes in .Net client application may be on start up or whenever but before starting your tests.
Moreover you can have it in app.config file as well like following
<system.net>
<connectionManagement>
<add maxconnection = "200" address ="*" />
</connectionManagement>
</system.net>