Shy or cautious fish can still be conned, sometimes - even when they're not actually feeding. Non-feeding barbel (and tench for that matter) are pretty impervious to anything you try.
I do agree that barbel are pretty stupid, which is a double-edged sword in some respects; on the one hand it means you don't have to be too clever to catch them when they are feeding, but on the other it means you can't really kid them to feed when they don't want to - unlike carp.
Not sure I agree with that. If a fish is not feeding its not feeding. I don’t buy into the idea that a Carp for example can be triggered into feeding while say a Tench or Barbel cannot.
Either they are not feeding or you used the wrong trigger. Just my opinion of course.