How do you deal with a not-so-great trai

I’m a training administrator who does more coordination of external trainers than in-house training. Most of the trainers I get are very good; sometimes they’re not so good. My question: how do you deal with not-so-good trainers? do you speak directly to them first, or skip them and appeal to the training vendor? I don’t like to wait until course evaluation time since the training is already over.