I suggest you use a multimeter to check the voltage between ENA+ and ENA- on your external drivers. It should be 0 when you start the Duet, changing to almost 3.3V when you first command the X motor to move.
Similarly, measure the voltage between DIR+ and DIR-. It should switch between 0V and 3.3V depending on which way you try to move the X motor. For example, if you send G91 first, then G1 S2 X10 will set it one way and G1 S2 X-10 will set it the other way.
If those are both working, then although most external drivers work OK with the 3.3V signals provided by the Duet, it's possible that your drivers need a full 5V.