Faith

Your teacher tells you to remain soft and loose, that you can move somebody simply by shifting your weight and turning your waist...
If you don't believe them, what happens?
You use local muscle tension. You seek to accomplish the task through incorrect means.

Your lack of faith in the tai chi and the teacher results in your failure.
Half-heartedness - trying rather than doing - is caused by doubt .
If you believe that you can move the person, you will use the tai chi.
Your mind and body must join to create intention.
If you commence a task with an attitude of failure, then you will fail because you have chosen to.