Start by examining the pins for the socket for the stepper driver that is failing. You are looking for pins not soldered. It is also possible it is a cold solder joint. The second picture in this post shows a pin not soldered on an SKR Turbo board.
If you don’t find anything, swap stepper drivers between one that is working and the one that does not. If the problem follows the stepper driver, then the issue is with the stepper driver. If it does not follow, then the issue is with the SKR board.
If it turns out to be the SKR board, it is possible to move the stepper driver to the 6th socket. This will require changing some pin assignments in the firmware and reflashing.
One additional thing you might try if you are comfortable, and if it is a control board problem, you could resolder the dir pin for the failing socket.