Issue resolved. We checked the network settings and everything was ok.
When I issued the command - admin tower list clients from the NSA server I could see all of the app servers listed.
The problem seemed to be to do with caching.
In our webfarm I took 1 of the app servers offline and restarted services to refresh the cache. Then I brought the server back online again and when we tested we were then able to see the portlet change as expected.
So I just repeated the above for the remaining servers and it all looks fine now.