Hello Agnes,
If you are pressing on the control and the machine is moving slowly it could be a worn out motor, first of all I would undo the foot control to make sure nothing is broken inside, these controls are easily replacable, to check the motor, hold the balance wheel lightly and then press on the foot control gently at first then increase the pressure, if you can hold the machine still without much effort then the chances are the motor has worn which will need replacement, when you hold the balance listen for the motor belt spinning round, it could be a simple thing like that.
Ken
drivewithken@blueyonder.co.uk, July 2007