This problem took me quite some time to figure out. To ease debugging (since I often forget to explicitly launch the debug-mode), I've enabled the Eclipse setting to always launch in Debug mode if there are breakpoints set. Unfortunately eclemma does not seem to work with the Eclipse debug launch mode, which means the Coverage view stays empty no matter how often you execute your coverage-run.
I understand that Debug mode and Coverage mode are not going to work well together since both require a java agent to be set in the JVM args, but the combination of the setting and the seemingly-working execution of the test (the breakpoints were simply not hit) leaves users easily confused why the coverage view stays empty. I guess some kind of FAQ entry should be made available in the documentation, unless there's a way that eclemma got enforce that the debug-jvm-arguments are not added to the launch..