Has this begun happening recently (after the Sierra update)?
If so this is actually something that should also be reported on the Apple forums so other people can work on it.
I found a similar thread here, but so far the only solution given was to revert back to El Capitan:
https://discussions.apple.com/thread/7694734?start=0&tstart=0
Most likely this is a new Sierra issue and not something we can help troubleshoot for you here.