It's actually been this way for as long as I can remember, sforeman. There has never been an assurance that ads will appear in top spots for any particular search.
Basically, ads earn their way to the top by meeting a raised standard in terms of relevance. If no ads meet the standard, then no ads are 'promoted' to the top.
Also, ads must be reviewed and approved before being sent to the top. So, if an advertiser who is in a top spot edits their ad, the ad will no longer appear at the top until reviewed and approved. And, once approved, the edited ad must still meet the raised performance standard in order to be promoted again.
Whether ads appear 'up north' for a particular search can vary from one search to the next, dependent on all the factors at play in the moment of the search.