I've found a few other threads on this matter, but all were pretty dated so I thought I should bring this up. The melee ability (Tempest Strike) for the Forsaken hunter arcstrider subclass doesn't seem to work when you have "hold to sprint" instead of "toggle to sprint." Other threads have suggested that changing the key binding of melee could cause the problem too, but as far as I can tell the ability works 100% of the time if you are on "toggle to sprint." This is rather frustrating as "hold to sprint" offers a lot more advantages than its counter part. The other threads I've found about this topic were all the way back in September when Forsaken came out. This should've been fixed long ago, so it would be great if a fix can come in the next patch.
The ability is based on the player sliding. If your bindings make it difficult to quickly activate sprint in order to slide you may experience trouble with sudden activation, particularly when standing very close to your enemies. If you confirm the player is sprinting before attempting to slide it will always work.