There's a lot happening - and changing - during the flare & hold-off, with an concomittent effect on the required control inputs. Not only is there a lot happening & changing but the
rate that things are happening & changing is also....erm.....changing. To make matters worse the rate of change is also changing.
No wonder it's difficult for the poor bloody student to perceive, interpret, decide & react to it all. I'm a firm advocate of limiting flying at this stage to calm or nearly calm weather conditions with a reasonable horizon *and* minimal turbulence. Think very early mornings....
You can help relieve the student's high workload by separating a lot of the tasks over time ie split the roundout, power reduction, hold off, landing & rollout into completely separate items. This will reduce many of the dynamically changing things that would otherwise occur.
First get the the student to transition to flying level over the runway. This will also help you judge his/her judgement about their flare heights & them to learn the control inputs needed to achieve level flight at the correct height over the runway.
Once they're able to fly stabilised S&L over the centreline have them (or even you, if necessary) *gradually* reduce power. Emphasise holding off & keeping straight while this is happening. Eventually the student will be able to get to idle power while still airborne & straight.
Get the student to continue doing more of the same while the speed reduces with the power at idle.
Then focus on control after touchdown etc