If you go into an interview with the perspective that your certification speaks for itself, you are probably cutting yourself short. If the gyms you are applying to are like most, a personal trainer is essentially a sales rep. It is up to you to find clients, convince them of your worth, grow and build a long term relationship. All the Kinesiology knowledge in the world is worthless unless you have a client willing to pay for it.
Perhaps it is time to work on your salesman ship and personality in the interview and elsewhere.