Google has added a new budget option, so that you can choose to either show your ads evenly over the course of a day, or a new accelerated option, which will show your ads as much as possible, until your daily budget is reached.
This will explain why sometimes your ads wont show up when you do a search. Its either that your budget has been reached, or your ads are just not being shown all the time.
Which ad delivery option is best for my campaign?
When you select a budget for your campaign, you can choose standard or accelerated delivery for your ads. This choice will affect how quickly your ads are shown each day, but in each case your selected budget will apply. To learn more about budgets, click here.
This option is automatically selected for you, and is best if you want your ads to reach a wide range of users over the course of the day. We’ll spread your ad impressions across the day to make sure you don’t accrue all of your clicks early on. However, if your budget is set below the system-recommended budget, your ads may not receive all possible impressions.
For example, if you want to spend US$30.00 over 30 days, your daily budget would be US$1.00. With standard delivery, your ads would appear throughout the day, from 12:00 a.m. to 11:59 p.m.. If your system-recommended daily budget is US$3.00, you may receive only one third of all impressions possible during that time.
This option is best if you want to increase the likelihood of spending your full budget each day. We’ll display your ads as often as possible until your budget is reached. If your budget is set below the system-recommended budget, your ads may not receive all possible impressions.
For example, you might choose to spend US$30.00 over the next 30 days, with a daily budget of US$1.00. If your system-recommended daily budget is US$3.00, you may receive all possible impressions until your budget is reached. Therefore, you could spend your entire budget between 12:00 a.m. and 8:00 a.m.. In this case, your ads wouldn’t appear again until 12:00 a.m. the next day.