This is a problem that is affecting me too and I have had several discussions with SmartBear support via email. One of our developers suggested that the problem could be related to the fact that SoapUI is using the Apache Soap Client and may be using this deprecated method:
http://www.soapui.org/apidocs/com/eviware/soapui/impl/wsdl/support/http/SoapUIMultiThreadedHttpConnectionManager.html#setMaxConnectionsPerHost(int)
If you set these values then they may not be taking affect and the default is 2 connections.
The SmartBear contact suggested that this may indeed be the problem but I've received no further responses and the nightly builds of SoapUI (Pro in my case) have not resolved the issue.
Dave
