[GeeTwo]

There is not a whole lot of FTC traffic on CD, but as I see it's been a while with no posts at all, I'll post something that may or may not be helpful.

Caveat: I have not done any FTC. I have used "generic" continuous rotation servos, however, e.g. the ones on Boe Bots. If the FTC continuous servos are not like those, this will be useless.

Generic Continuous Rotation Servos either do not have a potentiometer, or have broken the link between it and the rotation shaft. The practical upshot is that it is a "servo" that thinks it is always at center. This means that the farther from center you try to "set" its position, the faster/harder it tries to go that direction. There should be some "set" value which results in the servo not rotating.

When you change the "position", does the speed and/or direction of rotation change? If so, I suggest that you interpolate/extrapolate to where the wheel should not rotate, enter that value, and iterate until you get a non-rotating shaft. (This is known numerically as Newton's Method for finding a root, if you want to Google more about it.)

If the speed does not change, I'd suggest verifying that you're addressing the correct pin. If you're sure of that, try another method to see if you're really sending a good PWM signal to the port. This could be (for example) an oscilloscope, a regular servo, or a PWM motor controller like used in FRC. Hopefully the answer to that will give you a clue which way to go from there; if not, post your results and we'll help you through it.