Hi Steve,
it could be a variety of issues. In my experience it is most of the time related to the Java installation: Environment variables, Java version and 32bit vs 64bit.
But in general, Membrane Monitor is not under active development anymore. Have you tried Membrane Service Proxy? (
http://membrane-soa.org/service-proxy/index.htm ) It has the same core (although a newer version of it) and offers a Web Console which has almost the same functionality as GUI in Membrane Monitor. It is under active development, we and several of our customers use it in production and the standard features are pretty stable, I would say.
Best
Tobias