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The ads aren't showing and the Ad Diagnostic tool says I'm out of budget.
This could be a sign that the budget is far too low to support the traffic potential of the keywords in the ad group.
A slightly over-the-top example to illustrate the point: if each keyword is capable of delivering tens of thousands of impressions per day and an advertiser has hundreds of such keywords - all of which are supported by a budget of $1.00 per day. (This basic scenario is a surprisingly common, by the way.)
In such cases, the ad can show only a tiny fraction of the day, and the Traffic Estimator may indicate that the budget is in danger of being exceeded just about any time you check it.