Some terms don't display advertisements - if the combination of relevance and google's profit potential on a specific query is not above a certain level (which is always changing I'm sure based on yield management and user data tests), then no ads will display for that query.
By definition if no one has bid on the keyword, you can easily win the auction by bidding the minimum amount. It is likely that the terms you were searching for are not ideal targets for effective ads. Can you give an example of such query?
I quite often type in a search term for something that I imagine must be a very common search term and I don't get any adverts whatsoever.
Does this mean that nobody has bid on this keyword (because it's too expensive?) or are some search terms simply not allowed to be advertised on?