Show them that the learning is useful. Adults expect their learning to be practical and relevant. And they will best retain what they immediately put to use. So as soon as you've trained them, have them put their training to work. Reinforce the learning.
People remember what is reinforced.
That means you need to tell them frequently what they're doing well. You need to give specific instructions for things they need to improve. And you need to give them lots of time to use the learning on the job. The Selective Service might not be as selective as it once was, but, as you've noticed, adult learners can be very selective in their attitudes toward learning. Hopefully, if you follow these tips, their learning will be all it can be.