Starter motor/fly wheel issue

[DT RE Chris]

So,  my DT won't start using the starter motor, you can just hear the motor spinning but not turning engine over. 

the motor is fine and turns no problem,

the big cog behind flywheel(not sure what this is called) also turns freely when starter button is pressed,

the rollers on the back of the flywheel appear ok and move freely in and out, but just don't seem to engage,

Could the rollers just be worn down??

Can they be purchased separately? if so where?