Campanula and snails

Is there any reason why most of my campanulas survive contact with snails but some are massacred?

 

Because it's what they do....

Some are more resilient than others. The low growing, spreading ones tend to be a bit better in that regard, and the tall ones are usually more vulnerable to attacks. They often recover and come away again though, once the initial onslaught is finished.
I only grow our Scottish harebell [C. rotundifolia] now, and it's pretty indestructible. The others are a bit of a waste of time here as it's too hit and miss with them. Slugs annihilate them every year when they start into growth.

 

Thank you