Have you taken the plugs out, and checked that there is spark while the starter is not being used ?
Could be a wiring issue.
(Keep your hands well clear of the plugs while trying this)
We probably need more info.
Has it just lately been doing this, or you have never had it running ?
What happens if you try to blip the throttle while holding the starter button ?
Will it rev ?
This may not do the starter much good....