Here’s a few of the common reasons:
Too many searches
If you repeatedly search for your own business on Google but you don't click your ad, you may stop seeing the ad after a while. That's because Google's system recognizes your computer and stops showing ads that it thinks you aren't interested in.
We set our campaigns to spend the daily budget evenly throughout the day for the hours we have scheduled. This may mean if there are many searches for the many different services or products we are targeting that Google will hold your ad back from some searches to save budget for other searches that day.
You may be searching at a time of day or from a location that we are not targeting. We target our campaigns at customers when and where they will be searching from to find you.