Amazon Go, the cashier-less convenience store concept first teased by the e-commerce giant last year, has been in operation on a limited basis to company employees in Seattle since December.

The store uses an array of cameras, sensors, machine learning and advanced algorithms to track items as customers pick them up off the shelves. Rather than go through the traditional checkout process, you simply walk out of the store with your items and they’re automatically charged to your Amazon account.

According to a recent report from The Wall Street Journal, things have been going pretty smooth... so long as there are less than 20 people in the store and everyone moves slowly.

Amazon is also struggling to keep tabs on items that have been moved from their designated spot on a shelf, sources say.

The concept will be beneficial to both consumers as well as Amazon. Shoppers will be able to get in and out of the store much faster without having to go through a standard checkout process and without checkout lanes, Amazon wouldn’t need to hire nearly as many staff to tend to the store.

The technical difficulties have unfortunately delayed the opening of the first Amazon Go store for use by the general public. The Journal notes that Amazon had hoped to open its doors to the public by the end of March but it is now unclear when that’ll take place as the kinks are being worked out.