Iv cannulation can be tricky at times even for those who do it often.
I’m curious what kind of training the “volunteers with medical training” have as well… If they were trained for this specific purpose then its unlikely they’ve been able to practice on real humans, and practicing on a mannequin or IV trainer arm is very different from sticking a real human. Usually where I am, people who need iv training practice starting them in day surgery units. I can’t see a medical clinic agreeing to let someone from a prison come in and try to practice for the purpose of getting proficient at lethal injections.
Most health care practitioners have either oaths or a code of ethics that prohibits causing harm, hence why lethal injections are so often a mess.