Starter works fine but won't crank engine. the pick up has to be given a push first before it starts

If the starter is spinning when you try to start but doesn't spin the engine over then you most likely have teeth missing off the flywheel ring gear. The facts that you say you have to push it and then it starts means the flywheel position changes and the starter has good teeth on the flywheel to engage. To confirm this next time the engine won't crank over remove the starter and look inside at the flywheel for missing flywheel teeth.