Check these in Customer Orders:

• The order is unpaid or canceled — those checkouts never show on the KDS.
• The order is for another kitchen — the KDS only shows the kitchen currently selected in the admin.
• The order is not a Real Time order (the customer used the Meal Prep or A la Carte page).
• Its Real Time status is already Finished or Canceled (for example Uber reported delivered).
• The board was loaded before the order arrived — refresh the page; there is no automatic new-order polling that adds cards to the board on its own.