I think the problem with the St Ives branch is that the journey is so short, and there are so many people on the train that there is not enough time for the conductor to get to everyone to sell and check tickets, especially in summer when its 2 150s, which are full. What is really needed are portable ticket office buildings at all the intermediate stops. And sometimes you don't see any conductor at all, not even walking through the train.
I believe this used to be a problem on the Falmouth branch, but I think they have two people selling tickets (well they did a while back anyway).
The PROBLEM on the St Ives Branch is that tickets between St Erth and St Ives are ROVER tickets so don't seem to count to station usage.