AT&T's (NYSE:T) live TV streaming service, AT&T TV Now, has a problem. The app that subscribers use to access AT&T TV Now is, as of this writing, no longer supported on Roku's (NASDAQ:ROKU) eponymous platform.

That's a big problem, and it's only the latest in a line of problems for the live TV streaming service that once seemed destined to unseat industry leader Dish Network's (NASDAQ:DISH)  Sling TV and take live multichannel streaming services to new heights.

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Et tu, Roku?

For AT&T TV Now subscribers who use Roku devices to access the service, the new year came with some bad news. Support for AT&T TV Now's Roku app has lapsed. On AT&T TV Now's website, an explanatory note lays out the details:

Heads up: Starting Jan. 1, 2020, you won't be able to add the AT&T TV channel to your Roku device. Already have AT&T TV on your Roku device? You can keep using it as long as you don't delete the app. We're actively working on a new agreement with Roku and hope to resolve this soon.

This is very bad news for AT&T TV Now. By most measures, Roku is the most popular streaming platform on the planet (competitors like Amazon's (NASDAQ:AMZN) Fire TV sometimes claim to have passed Roku in one metric or another, but it's clear for the moment that Roku is still the industry leader). Roku boasts 27 million active users and a 39% market share that includes more than 15% of all connected TVs in the United States. And all of AT&T TV Now's major competitors offer Roku channels (Roku's term for apps): DISH's Sling TV, Disney's (NYSE:DIS) Hulu + Live TV, and Alphabet's (NASDAQ:GOOG) (NASDAQ:GOOGL) YouTube TV are all still available via Roku's Channel Store.

AT&T TV Now also lacks support for Alphabet's Android TV platform, something that is not true of its major rivals. But that long-standing issue can't compare to losing support for the most popular OTT platform on the market.

How did this happen?

AT&T TV Now's exit from the Roku Channel Store is the result of a lapsing deal and the inability of AT&T and Roku to reach an agreement on a new one. Like other platform companies, Roku doesn't let just any old app into its app store. Roku inks distribution deals with the apps in its store. AT&T TV Now was signed to such a deal, but no longer -- and, as of this writing, negotiations over a new one appear to have hit an impasse.

AT&T and Roku aren't saying what they're arguing about, but possibilities include a "platform tax" on subscription sign-ups through the Roku platform (a tech industry classic that is an infamous favorite of Apple) and advertising-related issues like ad placements and ad revenue sharing.

Is HBO Max a factor?

AT&T TV Now is not AT&T's only streaming service. The company also owns the soon-to-be-released HBO Max, an on-demand streaming service that builds on the HBO brand. There is only limited overlap between the two services -- AT&T TV Now is a live TV streaming service that offers live streams of network TV channels, while HBO Max will be an on-demand service full of original content that won't be available on live pay TV (streaming or otherwise) -- but the new offering could still be complicating things ahead of its Spring 2020 release. AT&T TV Now promotes two channel bundles that include a subscription to HBO (that's the regular one, not Max), which can turn into on-demand content via the HBO Go app. And HBO Max will almost certainly be launching on Roku, meaning that AT&T and Roku also have to work out the terms that will bring that app on board -- discussions that could bleed into similar negotiations over AT&T TV Now. For now, this is all speculation, as AT&T and Roku have been mum on this possibility.

The latest misstep for AT&T TV Now

The Roku debacle is the latest in a long line of problems for AT&T TV Now. In October 2018, AT&T TV Now -- then branded as DirecTV Now -- boasted 1.84 million subscribers and appeared to be on pace to topple Sling TV (which had 2.47 million during the same timeframe) from the top spot in the multichannel live TV streaming space. Then came subscriber losses in four consecutive quarters. The initial drop was reportedly kicked off, at least in part, by the ending of promotional accounts (discounted accounts which may have inspired overly optimistic assessments of DirecTV Now's rise in the first place), but that hardly accounts for AT&T TV Now's continued free-fall.

Taking on water, the service later rebranded and restructured its offerings. That hasn't done much to fix its problems.

With Sony's PlayStation Vue now officially retiring from the competition, AT&T TV Now is starting to look like the weakest multichannel service left on the market. While it has a decent base of more than a million subscribers, AT&T TV Now is trending in the wrong direction. Nothing illustrates the service's downfall more strikingly than the service's absence from the most important streaming platform out there. AT&T TV Now will likely be back in the Roku Channel Store soon, but even a temporary absence is likely to hurt the service's efforts to bring in new subscribers -- to say nothing of its reputation relative to its competition.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.