Precision 5520 will not sleep or hibernate

Hi everyone.

I have a Precision 5520 that SOMETIMES, only sometimes, refuse to sleep or hibernate.  It will always shut down, but sometimes when I click the windows icon (start) and go to sleep or hibernate, it just does nothing.  This isn't a huge deal, but it is sometimes very inconvenient to not be able to sleep or hibernate.  Does anyone else have this problem?  Any ideas for solutions?  it's been happening for a while, and I thought that an update would eventually fix it, but I guess I'm finally fed up enough to come here and see if anyone has any ideas.  Thanks in advance...