I have used a whip on the getaway from a fence before. It is not because of a poor jump, it is where I have used my leg and the horse ignored it. They may have got away with it at the fence in question, by me changing the plan and fitting an extra stride in, but at the next fence there may be a gaping ditch or something where it simply isn't feasible for the horse to ignore the go signal and still stay safe. The use of the whip would be after I landed and put the leg on, to give the horse a clear signal to go. If it is ignored, on top if the ignored signal in front of the fence, they will have a reminder to keep up to the leg.
I may well use the leg again before the final approach to a testing fence, and make sure there is a clear response.
Awful feeling, to be on a big course with a horse who is dead to the leg. If it is just the aid that needs refreshing then fair enough, but if the horse is tiring it is time to pull up. Sometimes it may need refreshing away from the other horses, or after a sticky fence.
ETA _ I am no way a professional eventer! It is certainly how I was trained by them though.
With some horses it would never be necessary. With one, when he moved up a level, it may be a couple of times on course until he settled into the rhythm of the new height/ more complex questions.
Far from lose confidence, it meant we had good approaches and smoother jumps.
It would be one smack, after the failure to respond to the leg. If it needed more, I would say that the horse needed to go back to a lower level or schooling.