In many instances ads will not appear on your videos. For example, if someone skips a pre-roll ad before it has completed, then this would cause it to count as a monetizeable view which was not monetized. This is because it was selected as appropriate to monetize but because the action could not be completed (viewer skipped the ad) it was not monetized.
Another example comes from users who use ad-blocking software on their browser, which prevents any ads from being shown.
Also, some countries are not appealing to advertisers so viewers from those countries may not see any ads even if a video is being monetized. Views coming from specific countries will not be monetized. To increase your monetized views, we suggest creating content that is attractive to viewers in countries where advertisers would want to show their ads, for example, but not limited to, countries in North America and Europe.