Remove the cover on the very bottom under the engine, you will see a wheel with rubber ( the friction wheel ) on it, when you squeeze the handle for the drive you should not be able to turn that wheel by hand. First check to see if there is enough rubber extending from the metal disk it is attached to, if there is then it might be just adjusting the drive cable till the drive disc grabs the friction wheel sufficiently. If there isn't enough rubber or the rubber is chewed up you may need to replace the rubber part ( if the disc has screws in it to be able to seperate) or replace the whole wheel ( if the disc doesn't come apart ).