
Take off starter motor and where the solenoid moves the starter motors clutch and pinion assembly along the motors shaft (where the cog slides along to meet the flywheel), give a good clean and plenty of WD40 spray to remove and thin oily deposits from this area.
Your solenoid can't move the pinion assembly to engage onto the flywheel and therefore won't engage the contacts to spin the motor.
This will do the trick and sort your problem!