Amazon’s Cashier-Free Store Might Be Easy to Break

Amazon needs a little more time to address some odd troubles it’s facing with its new cashier-free stores, according to a new report.

The online retail giant will not open its Amazon Go cashier-less stores later this month as planned, The Wall Street Journal is reporting, citing people who claim to have knowledge of its plans. Instead, Amazon needs more time to address some quirks it’s found in tracking customers at the store when they pick out their desired items.

According to the report, Amazon’s sensors are only able to track approximately 20 people in Amazon Go marketplaces at a time. Up to 20 people, the automated store can track customers and check them out without trouble. Once more customers enter the store or when they slow their movements, Amazon’s technology cannot so easily track them and the purchasing process breaks down, the report says.

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Amazon (AMZN) in December unveiled several brick-and-mortar store concepts as part of a broader push by the online retail giant to expand its presence in cities around the world. Amazon Go is among the smaller store concepts and acts more as a convenience store for customers rather than a larger grocery store. Amazon had previously said it hopes to open the marketplace in its hometown Seattle in “early 2017,” where it’s currently testing the concept.

The company is also reportedly considering drive-through marketplaces, as well as grocery store concepts. Amazon has already opened pop-stores in malls and a handful of bookstores across the U.S.

Amazon Go is designed to be a cashier-free market where patrons are tracked with cameras, motion sensors, and artificial intelligence to determine what customers are picking up and whether their purchase matches the items the technology believes they chose. If it all checks out, customers will automatically make a purchase through their smartphones without ever needing to head to a register and be able to leave the store with their items in hand.

The futuristic concept, which tries to take human employees out of the retail equation, is breaking down in a mock store Amazon has erected in a warehouse in Seattle, according to the Journal‘s sources. Employees act as the mock customers.

It’s unclear what Amazon might need to do to improve the system’s accuracy and track more than 20 people in a store at the time. But the Journal‘s sources say Amazon is anticipating significant interest for its Amazon Go stores and will not open the marketplaces until the technology is more sophisticated.

Amazon did not immediately respond to a Fortune request for comment.

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