what does the error log say ?
I've never seen the build button be greyed out (unless build automatically is enabled)
Good call! If I disable auto-build, then I can manually build the projects.
Looking at it more carefully now though, I see that I was just mis-interpretting the problem. It is building the project (in that it generates all the .class files), but it doesn't run the packaging at the end. Since it wasn't producing the final .war/.jar files, I assumed it wasn't building; but it turns out it's just not packaging.
When I clean all and build, the first, and most suspicious error it comes up with is this:
Skipping builder 'org.jboss.ide.eclipse.packaging.core.PackagingBuilder' for project 'Web Common'. Either the builder is missing from the install, or it belongs to a project nature that is missing or disabled.
... and it's right, that package doesn't exist; but I'm not sure what it expects me to do about that? I tried deleting and re-creating the workspace, and then cleaning and rebuilding all the projects - which changed nothing.
Is packaging supposed to be configured differently in this version?