Not to sound like a repeat of the previous posts, but ME TOO. ARGH.

Only, I have an idea about how the Snarbolax is stunned and how sometimes... he is not.

1. Check to see if he is "roaring" (meaning standing on hind legs and throwing head back... loud roar) when you attempt to stun him. I've noticed every single time I've hit the bell while he is roaring, it is ineffective.
2. Look for other actions that may be performing that is making him immune to the bell. I was unsure, but it seemed as though when he begins his three chomps attack, mid-chomp he does not get stunned.

I DID notice he can be (sometimes?) stunned while underground/burrowing if he's close enough to the bell, so try to use that to your advantage.

I have to say that even a pansy boss like Snarbolax should NOT at ANY time be allowed to be immune to the ONLY thing he's weak against.
This is why some people had to turn away and quit. This boss is quite easy, too. It's a HUGE waste of CE, and I think everyone who's posted here should get their refund of 60CE promptly. Of course, that's just what I think. I'm nobody's boss.

:P

~Chris-Son-Gohan