Walmart wants robots to run its warehouses. Reality keeps getting in the way, the Wall Street Journal reports.

The retail giant relies on more than 140,000 warehouse workers to keep goods moving. A worker might walk 20,000 steps a shift. Programming robots to do the seemingly simple job of sorting merchandise has proved stubbornly hard.

Walmart has spent a decade and billions of dollars chasing warehouse automation. Some early robotic tests failed or proved too expensive, creating a bottleneck as retailers race to speed up home delivery.

Rival Amazon leads the automation push and has now surpassed Walmart in annual revenue. The pressure to get robotics right has reached fever pitch.

The struggle shows that even simple human tasks resist automation. For now, Walmart's warehouses still run on people power.

Amazon now operates hundreds of thousands of warehouse robots. Walmart has tested robotic arms and automated storage, but scaling has been slow. Engineers say grasping oddly shaped items remains the hardest problem. The company says it will keep investing while keeping workers at the centre.