why won't my BodyForce car won't move on play?

Right, so I have a car which uses body-movers to steer and to move with a few problems.

seat = script.Parent

local vehicle = seat.Parent
local DriveForce = vehicle.MainComponent.DriveForce
local Turn = vehicle.MainComponent.BodyAngularVelocity
local speed = 40000
local turnspeed = -1.5

local engineSound = vehicle.EngineSound

function getSpeed()
    return math.pow(math.pow(vehicle.MainComponent.Velocity.X,2) + math.pow(vehicle.MainComponent.Velocity.Z,2),0.5)
end

while true do
    wait()
    DriveForce.Force = seat.CFrame.lookVector * speed * seat.Throttle

    Turn.AngularVelocity = Vector3.new(0,seat.Steer*turnspeed,0)

    print(DriveForce.Force.X)
    print(DriveForce.Force.Y)
    print(DriveForce.Force.Z)
    print("------------------")

    local gearthresh = 40 + (speed*2)
    local speed = getSpeed()
    local strain = (speed % 40)
    if (speed > 40) then
        strain = strain + 33
    end
    engineSound.Volume = (strain * 0.2) + 1
    engineSound.PlaybackSpeed = (strain*0.01) + 1

end

This script is inside the vehicle seat of the car.

It works absolutely fine when you play using Roblox studio's Start or Play buttons, however, when you play the published project, there is a problem.

When you press W (to go forward), your car won't move and appears to be stuck. I've checked the print statements and the force is definitely being applied to the car, but the force won't make the car go forward.

Interestingly, the car can turn perfectly fine.

Can anybody help? I can give more information if you need it.

Thanks!

-There are no error messages