If it that ESC you write about recently and the motor also you are playing with perhaps the combination needs calibration.
This is a specific set of steps, performed once.
See
for dets.
Also, it is helpful to just write a small program to get values from the serial monitor and publish them to the servo, you can sit there and play and get a feel for things.
You shoukd know how to get numbers from the serial monitor anyway, perhaps you already do.