In marketing, it’s settled science that it’s easier and cheaper to get an incremental dollar out of an existing customer than it is to get all new customer to buy or use a product.

If true, then why do mobile developers and publishers continue to spend disproportionately on driving app installs?

The answer has two parts: it can be harder than re-engagement on desktop, and many mobile marketers don’t prepare for it.

Last week, I moderated a panel on mobile re-engagement at Tune’s Postback event in Seattle. Joining me were three experts in this area: Adam Foroughi, CEO of AppLovin, David Philippson, Managing Director at Criteo, and Brian Malkerson, East Coast Sales at TapCommerce. What emerged is a consensus that re-engagement (or retargeting) is essential but often stymied.

Since re-engagement works so well on desktop, we might expect similar results on mobile. That’s not always true.

Desktop retargeting relies on cookies. On most browsers, these bits of code are pretty effective at helping advertisers know who has visited their site and what parts of the site they visited. That’s why we often see ads on other sites for products we already have viewed.

On mobile, though, cookies don’t work well. Also, most of our time on mobile is spent in apps, not the browser. Here’s where a mobile marketer’s lack of preparation can come back to haunt.

While it’s possible to track mobile app users even without cookies, being able to send those users into a specific part of an app requires something called deep linking. These deep links represent passageways through which marketers can channel users to a product item in the app, for example. They require coding during the app’s development, though, and so many apps may require rework before they’ll support deep linking.

Perhaps most formidable of all headwinds is measurement. Performance marketers have the mindset of a CFO. They relentlessly pursue whatever drives the most return on ad spend or lifetime value, and they will not hesitate to postpone or cancel an under-performing project if the money can be spent more productively elsewhere.

As you might imagine, persuading these people to divert marketing budget to a retargeting campaign requires that we have tools to measure changes in revenue tied to that campaign. Once again, when preparing your app for launch and lifecycle management, consider your plan for measuring the impact of re-engagement so that you won’t be caught flatfooted when the need arises. For example, a mobile marketer would be wise to proactively plan to use Facebook’s dynamic product ads even before launching an e-commerce app. Companies like mine and others can help.

There are other issues. For example, if a company wants to target a lapsed app user, it may be the case that this user has uninstalled the app altogether. A marketing message that intends to send that user back into an app presumed to still be on the user’s phone instead may send her back to the Store to download it all over again. That’s not a good experience.

As with most problems facing mobile’s adoption and maturation, the market almost certainly will address all these challenges. Until then, though, those practiced in desktop re-engagement might find mobile a hostile environment unless they prepare in advance.