Phew! Finally got this cannon Skill / Hero Point at Stormbluff Isle.  Easy now it’s working again.

Originally reported in the official GW2 Wiki as a bug since March 2015, all now appears to work via the instructional notes posted here.

As  notes indicate, you need to:

 click and HOLD the fire skill (#2) to elevate the gun. As you Hold a yellow horizontal bar progresses; the further right it goes before you release the button, the higher the trajectory.  I found that a full bar shot over the hilltop, that the higher targets needed about 75-80% full bar, the medium out near the sea about 50% and the guys that seem level with the gun about 25-30% full.  Easy!