CX has about 6000 FAs - however good they are at training, one or two less good ones are bound to slip through. I think the answer is to "name and shame" - from what I've heard even a single politely written letter from a passenger to Customer Relations about a named FA gets the FA in question a talking to; more than a couple and they are on the way out. On the other hand, positive letters also apparently have a significant impact on an FAs promotion prospects, so don't forget to recognise the outstandingly good ones too.

Personally I think over 8 years of regular CX flying I have praised about 8 cabin crew by name to Customer Relations, and only felt the need to name 1 in a negative way.