Are you actually playing a natural vibrato, or just attempting to produce an oscillating pattern in the tone?
First you must be able to spell it right.
Second, you have to be relaxed. If you are too tense, you can`t move your hand very easily, and you can`t vibrato.
I don`t know what kind of vibrato you`re doing, but always you have to be relaxed.
Most importantly though, it must be controlled. You have to be able to vibrato when you want to, at the desired width and velocity, and stop vibrato-ing when you want to.
Don`t get it so uncontrolled that you can`t stop. That would be kind of sad, and you`ll never be able to play Baroque music correctly.
