Hello,

I did a little internet searching and found that is a mixed bag of issues that can cause this. I snagged a link that is another forum from Apple where I found several potential fixes that might work. It seemed there was no one fix that worked for all of the people having this problem and has been an issue for a while. That link was https://discussions.apple.com/thread/1933163?start=45&tstart=0
I hope this helped.

Nick