My Gravely G16 did the same thing. First time it did this, I took a small hammer and TAPED the starter. After the second or third time, I took the starter apart and lightly greased the dry starter drive splines.
That corrected the problem. Dry grapphite would have probably been better since the grease will eventually be contaminated with dirt.
Over the years, I have done this with many engines--most people just purchase a new starter.