I've had a lot of problems with things not attacking the shield lately.

I usually send a pack of 4 Mini-Mole's to get the shield (they're fast and good at shield killing).

They usually destroy it in just a few seconds.
But I noticed I wasn't hearing the destruction of the shield when I normally do.

Tried it with a bunch of minions and nothing seemed to attack the shield.

And then... sometimes they do.

I don't do a lot of Verminion, though, so I didn't spend too much time on it.