This is a common misunderstanding clients have when it comes to Google Ads. After you set up the campaign, they head over to Google and perform a search for 'blue widget' or whatever keyword we're targeting in the Google Ad Campaign and then you get the ol' "What the hell!? Where's my ad!?" gripe.
There are many reasons why you or the client may not see his/her ad when they head over to Google and does a search. The main reason is mostly likely due to the budget not being high enough.
By default Google Ads 'Ad Delivery Method' is set to 'Standard Delivery'. This means it will attempt to 'evenly distribute your budget across the entire day'.
Obviously, this means that your ad isn't going to show EVERY TIME somebody in the given location searches for your keyword 'blue widgets'. You'd need a much higher budget to achieve 100% impression share.
With that said, I don't recommend optimizing a campaign based on impression share, because it completely ignores conversion and cost per conversion metrics. In other words, it's a great way to waste a lot of money.