Here are reasons why you might not be able to see a newly created ad:

Account Not Activated
To activate your account and begin displaying your ads, you must enter valid billing information. Your ads may begin to be displayed and accrue costs shortly after that.

Ad Editorial Status
All ads must comply with our editorial guidelines and advertising policies. If your ad is awaiting review, we'll try to complete the review process as soon as possible.

Ad Location
If your ad’s ranking is not high enough to place it on the first page of search results for a certain keyword, it may appear on a subsequent search result page. To see additional pages of ads, click "Next" at the bottom of the search results page, or click the "More Sponsored Links" link below the column of ads. If you want your ad to be displayed closer to or on the first page more often, you may be able to improve your ad’s position in search results by improving your ad’s quality and/or increasing your bid.

Server Delay
There may be a brief delay after you create your ads but before the time they are actually displayed. Sometimes it takes our servers longer than usual to upload and process your ads. Try checking for your ad again later.

Here are reasons why you may have seen your ad before, but can't anymore:

Daily spending limit
When you create your ads and select your keywords, you can set a campaign daily spending limit, which is the maximum amount you want to spend per day on your campaign. If your daily spending limit is set lower than the amount necessary to receive maximum traffic, we may slow or even stop the display of your ads for that day to stay within your daily spending limit. To maximize your potential traffic, consider changing your daily spending limit to meet the recommended amount.

Declined Ad
If an ad doesn't meet our editorial guidelines, it will not be eligible for display and its editorial status will be shown as "Declined" (or "Removed," if it had once been approved for display). You can view declined or removed ads and keywords in the Editorial Status page, which is located under the Campaigns tab. Here you will be able to find any declined or removed ads in your campaigns and ad groups. If this is the case, please make the needed edits and resubmit your ad to us for review.

Ad performance
A low quality index score may cause the ranking of your ad to drop down in the results. As a result, you may not see your ad in the same place or page as before. To monitor your ads' performance, go to the Ad Group Details page.

Please note that any ad with a low quality index may also be in danger of not being displayed in search results. We may filter out low quality ads to improve the search experience of our users. Our analysis has shown that improving the collective quality of ads shown for a particular search term improves click-through rates. As a result, we recommend that when you see that an ad’s quality index is low, you take immediate action to improve its quality.

It is also possible that your ad is not being displayed due to a combination of a low bid and low ad quality. In this case, it is possible that a bid increase would result in the ad being displayed. But always focus on improving your ad quality first. Higher quality ads may allow for a lower bid and/or receive better placement on the results page.