Technical difficulties have delayed the public opening of Amazon's futuristic new grocery store.

The store, called Amazon Go, uses machine learning and cameras to detect what's in your cart and automatically charge your Amazon account so you can leave the store without ever taking out your wallet.

But Amazon Go isn't quite ready for primetime, according to The Wall Street Journal's Laura Stevens. The technology is having trouble keeping track of more than 20 people at a time and struggles to track an item that has been moved from its place on the shelf. Right now, things only run smoothly when there are just a few customers in the store or if they're moving slowly, the Journal reports. 

Amazon has been testing the store at its campus in Seattle, with employees serving as beta testers. Amazon officially announced the store last December with a video showing how it will eventually work and said at the time that the store would be open to the public in early 2017.

Amazon did not immediately respond to a request for comment. 

This post originally appeared on Business Insider.