Some WADS, notably Google and Bing use a quality metric called a Quality Score. If your score isn't at least 8/10 you may need to write better ads, modify the landing page to be more tightly themed to the ad, or put the ad into a more thematically related ad group.
If you have a quality score of 5 or less, start over from scratch. No kidding, trash it and start over because your campaign doesn't meet their quality standards. If your quality score is 6 or 7, it usually comes down to a few things:
This is simply a matter of bidding more. It may take nerves of steel at the beginning, but hang in there. Slowly increase your bid price and check back every hour or so to see your positioning. Newbies often come into a space with crazy high bids, but soon give up if you hold your position for a couple of days.
The keywords you're bidding on do not match the ad wording closely enough. In this case write new ads that include the exact keyword phrase you're bidding on, in the ad title, and the ad copy.
The other way is to remove the poorly performing ads, and paste them into a new ad group, that is more tightly themed between the ad group name, wording in the ads and the keywords you're bidding on.
This is when the search query more closely matches the ad. So lets say you're selling farm sinks and your ad displays for that phrase. If the search query is stainless steel farm sinks, someone bidding on that exact phrase will have more relevance than an ad for just farm sinks.
Therefore, it's a good idea to look through WADS history, to see what search phrases people are using when they click on your ads. So if you're selling farm sinks and bidding on that phrase, the WADS may show 40% of your clicks are coming from people typing in copper farm sinks. In this case, create a new ad group and ads, specifically for copper farm sinks to get more relevancy.
It's the same deal if you notice that a majority of clicks are coming from a certain geographical region, like a state or province. In that case, create a new ad group, ads and lander, with the name of that region in the ad group, ads and lander.
The ads do not match the landing page closely enough. If you have control over the landing page you can change it to match the ad, if the ad is performing well and getting clicks. If you cannot change the landing page, change the headline and copy in the ad to better match the lander.
It also helps if the page loads fast. If it's your page you can always optimize the images, compress the code, etc. If it's not your landing page and it's painfully slow when loading, it may be best to find a new offer from a different merchant.
Click Through Rate
If your ad keeps getting impressions but no one clicks, then the ad will have a low clickthrough rate, giving it a lower quality score. The only cure is to write more relevant ads that get clicked on. Often its just a matter of adding a call to action to the end of the ad like; Learn More, Shop Now, or my favorite, Buy Now!
Before you can have a share of market, you must have a share of mind.