Using the appropriate voltage for a servo.

So my friend and I are building a simple robot that should just be able to roll around. We have 2 different sets (of 4) of continuous rotation servos I can't recall the exact voltage for them, but some are around 7 volts and the others are ~4v. We've been using a 9v battery to power the 4v servos, and it works fine, but for some reason when we try using the 9v battery with the higher voltage servos, the servos never stop turning. We're both new to dealing with even the simplest hardware things, but we are pretty sure all of our code is right. Any advice on what the problem could be? The main reason we would like to be able to use the larger servos is because we are not sure the smaller ones can support our robot.

Thanks.

...and 9V batteries can't support your larger servos.

Get a proper battery that's going to be able to supply the current required by those servos. LiPo, NiMh, NiCd are all good. AA alkalines are also really good.

but we are pretty sure all of our code is right.

You have made bad assumptions in one or more areas, code could be one. Do you have info links to the servos you have? Continuous rotation servos usually need to be "calibrated" to not move at a 1500us command position.