I think some of it's mental, but sort of in a different way.
Rondo's primary job is to get everyone else shots. He does it very well. Amazingly well. But that's his first mindset when he catches the ball -- pass it or dribble with it. So when he catches it 15 feet from the hoop an is wide open, he's not in position to shoot. He's in position to look for a pass, and in the mindset to look for a pass. He isn't squared up to the basket, and hasn't had his eyes locked on it ready to go.
Contrast that to Ray. When Ray gets the ball, his first inclination is to shoot. If he doesn't have a good shot, he might pass it up, and a small fraction of times when he catches it he's already looking to pass it again immediately. But by and large, he's ready to shoot, and that helps.
In practice (I imagine, as I don't watch all of his practices,) Rondo is passed the ball from an assistant, and he shoots. In games, he's passed the ball from a teammate, looks around at a couple players to see who's open, maybe dribbles a couple of times, and then shoots. It's a completely different experience. What he should do when practicing his jump shot is to catch the ball, look away from the hoop for a half second, and then take the shot. Shooting is all about muscle memory, and that's why Ray is so good at it. But Rondo can't try to develop Ray's muscle memory, since his in-game experience is different. He needs to practice the repetitions that he experiences in games.
Lastly, I doubt he'll ever be a great shooter. If every basketball player could become a great shooter, they would be. But he can become an effective shooter, and considering how great the rest of his game is, that's all he needs.