Don't overthink this one, just watch their hips. If they don't come up first out of the hole, you've got a problem. Sometimes this takes awhile to sink in. Once you put a bar on their back, some people forget everything. Be patient. Use tactile cues and touch their low back. When they get to close to the bottom, remind them with a cue like, "Hips up!" You may have to yell that every time until the internalize the movement.
I do this too for difficult cases. Be very clear, "The only thing I want you to think about is driving your butt up. I don't care what else happens, your butt comes up first. Exaggerate the movement."
Sounds like you're on the right track. The more people you deal with the better you'll get at picking the right cues for the right type of trainee.