Often, the Google algorithm can filter your ad out of your feed due to irrelevant search history or other factors that it deems not appropriate for you to see. You might also have reached your daily budget, which means the ads are only scheduled to show up at certain times of the day, and you're unable to view them.
One of the biggest reasons is that if you are searching for your ad with the same IP address you use to manage your Google Ads account, Google might avoid displaying your ad to yourself to prevent performance metrics from being skewed.