The most common reasons: the order's delivery method has Export to Root Planner? switched off; the order is a pickup (pickups are exported through the pickup location instead); the order is fully refunded/cancelled; the order is an event/proposal order (never exported); or the order already has a tracking code from an earlier export (then it is listed under Orders Imported). Also check that you are viewing the right kitchen location — with a location selected, only that location's orders are processed.