There's a multitude of reasons why this could happen such as a temporary interruption of work or jobs that took much longer than originally projected. Principally BOINC should be fetching work no later than when the last job is completed on a multicore. With version 6.10 better logic is implemented to make sure all cores remain busy, irrespective of the progress and efficiency statistics. WCG is working to adopt this client and will promote it to recommended when it has fully stabilized. Until then just let it run, and if there is no work at all being processed to reset the project in the client.