Did that with my first starter and it started but not all the time.
Then I brought that stater to a guy that specializes in started and I bought the kit with the new drive and new contacts and he tested start and it worked perfectly, then went to put it in my bike and got the same behavior that's when he said perhaps the gear is a little worn, so that when I bought this other starter and still have the same thing happing, so now it sounds like for what ever reason it's must not be getting the full 12 volts to start? Maybe it's the spark plugs I will check them.