Amazon’s first cashier-free convenience store was supposed to open to the public early this year, but apparently some big technology issues have the launch on hold.

According to The Wall Street Journal, Amazon’s Go store currently only functions if there are fewer than 20 shoppers inside. Any more than that, and Amazon’s shopper-tracking technology breaks down, as people become too difficult to follow. Amazon is reportedly having trouble tracking products that are moved from their proper spot on a shelf, too.

Amazon had been hoping to open the store by the end of March, according to the Journal. But the launch is now being delayed, and it’s unclear when the store might be ready to open. Amazon didn’t immediately respond to a request for comment.

This is a setback for Amazon as it begins an expansion into retail stores, where it’s looking for something beyond a recognizable name to set itself apart. Amazon has already opened five bookstores and has plans for five more, but it’s this next generation of cashier-free stores that would really distinguish the company’s retail strategy.

Amazon is trying to build stores that would let customers walk in, pick up items, then walk out and have them automatically billed to an Amazon account without any need to wait in line. It’s apparently considering an expansion into grocery and warehouse-style stores, too, though it’s not clear if these would use the same shopper-tracking setup.