If it's not a physical issue, it's likely between the ears, like Rusty said, assuming obvious equipment issues can be easily addressed.
It's really easy to fall into the "good enough" trap - and it's often a subconscious issue. We are conscious that we can and need to improve, but sub-conciously may think "meh, it's ok - I'm not doing that bad and I'm having fun".
I've been stuck before, for a season or more, content with my skiing, before changing it radically again and again, as my eyes opened to new ideas... and yes, now trying to change it radically, yet again... although to the untrained eye, it may not look like it.
Range of movement is a really big issue and I find internal cues for that unreliable - my own experience with that. Skiing by feel or contentment can only get you this far... video doesn't lie and helps tremendously, but it can be demoralizing and you need to find a way out.
Drills help, but if not pushed to the extreme, there's the risk of settling even deeper into that "good enough" range of motion plateau.
To change the skiing, they must change something... and again, range of movement of different movements is one thing to focus on. As a coach, you could find the extremes of the different movements and target that: flex this much. Incline this much. Angulate like this at the apex. Coil this much between turns... etc... and as the skier starts to work on those ranges of movements, he will start changing.
The other thing to realize is that change is not easy. My skiing has completely fallen apart many times when going through big changes. And it continues to do so. The skier will be out of his/her comfort zone and must be willing to put up with it for the day/days that it takes. There must be trust that the direction is correct and that even if the skiing is completely out of whack, there's an expectation of improvement.
p.s. Skiing this weekend?