With horses and students this is often the case if they have been training previously with someone else.
You may clearly give your horse the correct classical aids to lengthen the trot . . . and then your horse canters.
You may ask you students to stretch down with their lower leg . . . and then their heels are forced down and their lower leg swings forward.
With both horses and students . . . lower the level of expectation, create the basic groundwork and reward them when they ‘get it’ with a simple ‘good boy’ and a break or positive feedback for your student.