The most likely answer, based on what you posted, is that this is a budget issue.
Since an ad may theoretically be clicked up to 2 hours after an ad impression is generated, there is often impression throttling to prevent your campaign from exceeding its daily budget. Once enough impressions have been served, that could be clicked at some point in the near future, the system will throttle down impressions until enough time has passed to safely start serving impressions without exceeding your daily budget.
The primary function of the daily budget limit is impression throttling. Even though your daily budget isn’t being exceeded, it will certainly cause impression throttling unless it is set very high over the potential traffic for your targeted keywords.
I typically set my daily budget to at least five times what I plan to spend daily to prevent impression throttling. I prefer to control my ad spend by bid levels rather than impression throttling. The side effect of this tactic is that it tends to lower my CPC significantly.
