It can be a very frustrating feeling to set up a campaign only to see it serve very few impressions. The following topics are the most common reasons why your impression count is smaller than you expect it to be.
In a nutshell: If your campaign isn't serving enough impressions your audience is probably too small or too hard to reach, your ads might not be approved, you might have goofed up your campaign schedule, you might not be bidding enough, or you might need to upload ads in more sizes.
Take a look at the articles in this category for campaign troubleshooting tips.