Starter problem

[roadster]

I have never had this before.
I was bumping the starter from under the hood .
it stuck and kept turning over until I tap the switch
changed the switch to a new one .
still does it.
looked like knob had some spot on it so I sanded the
brass colored knob and still does it.
the switch is sticking to the knob until I tap switch.
what is this?