Shield not working, am I missing something?

HI all. I’m observing that, very often, my shield is not working. The model shows its shield being up, but the attack ignores it outright. I know that some attacks are shieldbreakers; when this happens, the shield doesn’t block any attack at all.

This was especially frustrating against the undead dragon. Some time ago, we killed it reasonably easy by blocking the flame; this time, we did exactly the same things, but the flame bypassed our shields about 50% of the time, resulting in a wipe rather quickly.

Is this a bug or are we doing something wrong?