Source: Apple.

There's no debating Apple's (NASDAQ:AAPL) new iPhone 6 and 6 Plus are a huge success. Just this week, the company announced it had sold 10 million of them in just three days.

But while there's plenty for the new iPhones to brag about, Apple's virtual personal assistant, Siri, appears to be slowly falling behind the competition. Even with two new models and an iOS 8 update, Siri comes up short against the competition.

What Siri lacks
Despite being location-aware, Siri has never been able to match some of the more useful location features Android phones enjoy through Google (NASDAQ:GOOG) (NASDAQ:GOOGL) Now. And now Microsoft's (NASDAQ:MSFT) new virtual assistant, Cortana, has Siri beat as well.

While Siri can remind you to do something when you arrive or leave a specific location you frequent often -- like your home or office -- it can't remind you do things like pick up bread when you pass any grocery store like Cortana can.

Even more useful is Cortana's ability to remind you to do, or say, something specific to a contact when they call, text or email you. So if you have a question you need to remember to ask your boss, just tell Cortana to remind you the next time your boss contacts you, and she'll do it.

And it's not just Siri's basic reminder skills that disappoint, but also the fact that she's not always listening to what you have to say. Take for instance the Moto X. It has a co-processor that allows Google Now to constantly listen for voice commands. The Moto X also learns a user's voice, so it can distinguish it from other people talking around it.

With iOS 8, Apple added a similar function that allows Siri to listen all the time, but only when plugged into a power source. Obviously that's not ideal for a smartphone that's unplugged and mobile most of the day. Apparently Apple thinks the feature would draw too much power if Siri was listening 24/7. 

Making Siri smarter
Beyond the "Hey Siri" listening feature, Apple's virtual assistant can now listen to and identify songs. Siri also pairs with Apple's HomeKit platform to manage home automation voice commands. It's a nice feature, but not one many people are using right now.

To truly make Siri a standout feature in the future, Apple will need to designate more hardware power to the assistant and bring Siri in-house. Over the summer, rumors popped up that Samsung may purchase Nuance, the company that makes the back-end technology for Siri. That sparked some discussion that Apple needs develop the feature more internally.

But to truly make Siri smarter, Apple needs to follow in the steps of Google and Microsoft. Both companies use what's called "neural networking" for their virtual assistants, which helps the phones better understand the context of what's being said, not just the words. Siri doesn't have neural networking right now, though Apple's reportedly hiring people and building out its own team to make it happen.

Foolish thoughts
Upgrading Siri won't change Apple's bottom line, and it's not as if Siri's holding back the iPhone 6 and 6 Plus from being massive successes. But with Google Now and Microsoft's Cortana outpacing some of Siri's capabilities, its time Apple started putting a little more effort into its virtual assistant. 

If not for the iPhone, than Apple should make Siri better because of its new watch. The Apple Watch firmly placed the company into the wearables market, and that means voice dictation, recognition, and contextual awareness will be more important than ever.

Apple sells some of the best premium mobile and wearable devices on the market; is to too much to ask for its virtual assistant be one of the best as well?

Chris Neiger has no position in any stocks mentioned. The Motley Fool recommends Apple, Google (A shares), Google (C shares), and Nuance Communications. The Motley Fool owns shares of Apple, Google (A shares), Google (C shares), Microsoft, and Nuance Communications. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.