Further to my above answer,
check all connector blocks and make sure they are clean and not corroded if they are clean with emery paper or replace as needed. it might also be the motor brushes that are not making contact properly, these are located on the sides of the motor under a small black cover about the size of a UK 1p these unscrew and the brushes pull out, there is a spring attached that pushes the brush against the motor, take them out (one on ether side of each motor 2 in total to each motor) and clean with alcohol or similar, be careful not to damage them, as they are out use a Hoover to suck out any dust particles out of the motor housings, just put the tube over the holes you removed the brushes from and suck. then back to the brushes, when they are clean and dry replace them back in to the slot you removed them from, do not force, they should slide back with ease you need to place the spring and the gold cap back in and hold it there as you place the cover back and tighten up the covers, you will need to turn the spring cap around until it slot in. see picture
if this helps you please click "YES" button
good luck,
Mark.