You're marketing your business or service through search and social-media platforms, running pay-per-click ad campaigns across several of them. Your ads are carefully targeted to appeal to your potential customers, but you're not getting the results you were hoping for. Or you're getting decent results but you'd really like to do better.

Maybe the ads are fine. It might be your approach to the different advertising platforms that's the problem. That insight comes from Jane Harrell, founder of 'cause Digital Marketing. In her years in online marketing, she's seen a lot of mistakes that rookies and even seasoned executives can make. Avoiding these can help save you from wasting money and time on digital ad campaigns that go nowhere.

1. Picking the wrong platform for your product.

Don't just choose the platform your audience uses most; pick the one that will work the best for the type of product or service you're offering, Harrell says. "Some people think that Bing, Google, and Facebook ads should all be the same, so they just throw something at the wall and spend a lot of money putting it on all those platforms," Harrell says. Instead, she advises, pick the platform that makes the most sense for what you have to offer.

"Does your product or service solve a problem that people are looking for solutions to? That's a great reason to use a search platform such as Bing or Google Adwords." Though Google has been the market leader for years, Bing has been growing steadily, she notes.

If your product lends itself to demos or if you have something to teach your audience, consider creating a video to promote on YouTube. "Most entrepreneurs don't realize that YouTube is the second largest search engine," she says. "It's a great platform, and a lot less expensive."

On the other hand, if your product isn't something people search for and it doesn't lend itself to video but you're trying to engage people's curiosity, then consider buying ads on Facebook or Instagram, she says. Buying ads is now the best strategy for Facebook, she adds. There was a time when a popular company could build a large following organically and then reach all those people by simply posting content. But these days, even if a large number of people have liked your page, you posts probably won't get seen by that many of them. So rather than asking your audience to like your page, use your Facebook advertising dollars to collect email addresses, so that you can send them content directly.

2. Trying to drive traffic to a competitor's platform.

You've created a great video for your product and posted it to YouTube. You want lots of people to see it, so you buy some ad space on Facebook. Bad idea. "It's expensive and not effective," Harrell says. Although the major platforms won't say how their algorithms prioritize one ad over another, she's observed that ads that attempt to send users to a competing platform often don't perform well. 

"Promote your YouTube video through Google AdWords and upload that same video to Facebook and promote that through Facebook ads," she says. "That way you're having people stay on their platform."

3. Targeting your audience with too many specifics.

Of course, it's possible to be too general, targeting an audience of everyone in the Midwest, for instance, or trying to buy a very general search term, such as "dog food." If your search term is too general, your ad won't get seen because large industry players with deep pockets will simply outbid you. 

But Harrell says the opposite error is much more common. "Some people will use a sample size that is so specific--65-year-old men retired from the naval industry," Harrell says. "Or using a very specific search term. Across the board, we see having a slightly broader target audience is typically more beneficial."

Facebook has a gauge that shows you the sample size for the parameters you're setting. "We shoot for about 2 o'clock on that gauge," she says. On search platforms such as Google, look for a collection of search terms that total at least a few million searches per month. "You can use a long tail search term such as 'best veterinary practice in New York,'" Harrell says. "But combine it with other terms such as 'NYC vet.'"

4. Bidding against yourself.

This can happen, Harrell says, if you run two versions of the same ad on the same platform targeting the same audience, but the ads have two different calls to action. For example, one encourages people to visit your website, while the other asks them to sign up for an email newsletter. "Essentially, you're bidding against yourself and driving the price up," she explains.

Instead, she advises having one call to action, but using two or more ads so you can see which works best. Some platforms will allow you to A/B test two ads. If not, manually switch ads from time to time to see which performs best.

5. Leaving the same ad unchanged for too long.

"We see a lot of small businesses create an ad and run it for a while," Harrell says. "They're happy with the results so they don't log in again for a month or more. But all these platforms work best when you look at it and tweak it at least weekly. You'll be able to see when an ad that was performing well starts to lose efficacy."

Sometimes there's a new competitor or a change to the platform you need to be aware of, she says. And time of year can make a big difference as well. In fact, for some companies, it makes sense to skip running digital ads in the last three months of the year altogether, when big online retailers bid up ad prices as they seek to woo holiday shoppers.

"Make sure you're looking for time-of-year fluctuations," Harrell advises. "Don't set it and forget it--ever."