There are many reasons, some mechanical, some electrical, why a vehicle won't select four-wheel-drive when the driver requests it. But the first thing we'd check would be the electrical connections from the rotary dial on the dashboard and the mechanical components that dial controls. In the old days, four-wheel-drive was selected by mechanical levers that moved the internal bits of the transfer case to the correct position to engage the front driveshaft and, in turn, the front differential and axles. But in your Pathfinder (and many other makes and models) manufactures started using electrical switches instead of actual levers. The idea was that by electrically controlling this function, there was less effort required from the driver. The reality is that these dials are as susceptible to electrical gremlins as any other part of the car. Check the fuses that protect the selection dial and if that's not the problem, have the rotary dial itself checked for faults.