I know, I make a lot of bug posts, but I figure you guys actually read this forum quite frequently and thus:
The reason so many folk have trouble with, and report about, our beloved Snarby, is that if you hit the bell while he is in the middle of an animation [the Bite-charge up for example] it will not remove his invulnerability. I've been chased around by his five linked bites before and hit the bell twice during a single [albeit extended] roaming stage, but didn't de-cloak him once, despite him always sitting directly atop the bell.
Got him in the end, but it drives me a little bit mental is all.
Yeah, I've seen this many times. If he's in the middle of an animation... he won't stun. Which is pretty lame, as usually the only time he's near the bell is when he's attacking.