Amazon.com Inc. is testing its cashier-less checkout technology for bigger stores, according to people familiar with the matter. If successful, the strategy would further challenge brick-and-mortar retailers racing to make their businesses more convenient.

Amazon’s existing stores are the size of a small convenience store — and are thus much smaller than your typical grocery store, with fewer items and people to keep track of. Amazon’s rollout of the stores was delayed in 2017 because the stores kept breaking when there were more than 20 people inside. The Wall Street Journal says that Amazon has improved the software in those stores since they’ve opened, but apparently, the technology still has trouble in those “bigger spaces with higher ceilings and more products,”

The online retail giant is experimenting with the technology in Seattle in a larger space formatted like a big store, the people said. The systems track what shoppers pick from shelves and charges them automatically when they leave a store. Although the technology functions well in its current small-store format, it is harder to use it in bigger spaces with higher ceilings and more products, one of the people said, meaning it could take time to roll out the systems at more larger stores.

It allows shoppers to buy things with the help of a smartphone app by tracking what they pick up from the shelves and what they put back, billing their credit cards after they leave.

The company is understood to be planning to have around 3,000 of the cashier-less stores by 2021. It is not known where or when it will be opening its first UK cashier-less store.

Amazon did not immediately reply to a request seeking comment.