Originally Posted by JRA
This is most likely from the index not having finished updating itself yet since it runs slowly in the background when the computer is idle or in low use. You can rebuild the index to force it to update now. This will show you how to.
Another option is to disable the index to have accurate search results all the time. The only drawback is that it will take a little bit longer to run the search. You can always enable the index again if it is just to slow. This will show you how to enable and disable the index.
Index - Enable or Disable