Does this just not work unless you're an axe spec?

I've spammed it to 100% on previous characters and never seen a single success, I had attributed that to using a wooden axe on a metal shield. However, using a metal axe on on a metal shield, having used the skill around 50 times I've still never seen it succeed. These are low-level shields on low-level mobs.

What's the deal?

Dunno, works fine in testing for me. What message are you getting? If you get the 'holds firm' you can't cleave that shield, but if you get 'narrowly misses' it means you just failed your chance.

You get a bonus for being axe spec, but it isn't required.

My current has 100% in the skill again (goes up quick when you're learning from mistakes every time!) so I've decided to stop spamming it but I never got any other echo or saw a broken/damaged shield.

Still, I must have used the skill well over a hundred times without seeing it succeed at all.

Go try on kua toa guards in the tunnel. If you don't have close to 100% success cleaving their leather shields then something is broken.

Send me a note in game and I'll look out for you, and see what's going on.