Hi,
make sure that if you signal your execution to move forward you need to take the processInstance object that is returned by that method. Otherwise you will work on old reference.
If you are working with tasks you most likely need to run following method after you complete your task:
procesInstance = executionService.findProcessInstanceById(processInstance.getId());
And after that you invoke findActiveActivityNames and you should get correct results.
Cheers,
Maciej