What your describing is the melee bash, it works as an interupt, but you need to be up close & personal with whatever you are bashing/interupting

It doesn't matter what weapon you have equipped it is your actual proximity to the enemy that matters

if you are looking for a range interupt, you will need to use a slotted skill

im going to try & explain this in written form (which is not easy 4 me im a talker not a typer, lol)
I apologise if it comes across patronising but, well see above sentance.

so when you hold LT down, gold summond shield should appear infront of you
when that is up doing a light attack or pressing RT quickly, not holding it, will do the bash animation

alterativley doing LT & RT light attacks at the same time should do the bash animation .the timing can be glitchy if you are experincing lag, which is why many people prefer the 1st way

you need to be right up in the face of the player/boss/npc that you are trying to interupt.
you need to be 'locked on' to them (they are glowing red)
you can bash at any time & deal some damage, but you can only interupt them when they are casting a skill

please note some boss monsters CAN NOT be interupted, that is intended dynamic of that dungeon/event,
and some players in PvP run immovable pots, or the HA skill which grants them imunity to snaring effects which includes interupt


Now i dont know the in's & outs of this next part,
i had a tank, but i never played the interupt build, i played the stand there & take a beating/keep everything taunted build, so i didnt pay as much attention to interupt/immunity mechanics as i really should of
but i believe there is a cool down on how often you can interupt, the boss/npc/player will get a white swirl around them when they are imune (whether from immovable or from the cool down)
when this is up attempting to interupt would be a waste of resources.