I was able to reproduce this problem with unit-tests in activiti-spring - simple replace in CustomTaskAssignmentTest.testSetAssigneeThroughSpringService.bpmn20.xml line
and you will get error like "no process definitions found in deployment" - process simple is not deployed.
Some notes what may help to fix it:
for activiti:assignee it works with any number of arguments
for activiti:candidateGroup it work if assignment handler method as 1 argument - it is not working in case it has 2 or more arguments