The fact they don't practice penalties probably explains a bit. I know you can never recreate the high pressure situation and it probably doesn't make it a thing but I'd still have them practice it. Good on Roy though. At least he's recognised his short comings in this department.
It's absolutely flabbergasting that they don't. Developing the muscle memory and ability to hit, say, two different spots 10/10 times makes a huge difference in a game situation.
Also always a good idea to have more than one player practice them, you can never rely on the one designated player always being on the pitch when you get awarded a penalty.