There are a lot of possible reasons.
The most common is misspellings. Example: Shackspeare instead of Shakespeare.
Another problem is too many details. In database searching - "Less is more."
Try this technique:
Instead of searching for:
Wolf, Alison. Does Education Matter? : Myths About Education and Economic Growth. London: Penguin, 2002. Print.
Remove everything except the author and title.
Then remove all punctuation.
This works Wolf Alison Does Education Matter Myths About Education and Economic Growth