Easy to overlook, but you should include ellipses (...) after a package name, to also enable assertions on it and all its subpackages.

$ java -? 2>&1 |grep -A2 -e '-ea'
    -ea[:<packagename>...|:<classname>]
    -enableassertions[:<packagename>...|:<classname>]
                  enable assertions with specified granularity

So, use -ea:com.foo... to enable assertions on com.foo and all its subpackages.