Java Client access to DNS

More of a Java/DNS question really I expect.

Our Java Client application allows you to enter a server to connect to 
during login. So you could enter something like:

http://www.bigserver.com

However, we have one client for which this is failing to do the DNS lookup. 
If we enter the correct IP address in place of the domain name then it 
works fine.

They do have DNS access from that computer because entering the same domain 
name in a browser takes them to the web page ok.

So my question is, what could stop the Java Client app being able to do the 
DNS lookup when the browser can?

Thanks
John