Mine did the same thing.
I changed the starter and found it wasn't't long before it wouldn't't engage properly so I tried a little "Morey's" oil on the gear and shaft.
If you have never heard of it, it's a very heavy and sticky oil additive.
Any heavy gear oil should work but remember don't use too much just a couple of drops.
For some reason "Morey's" worked better than grease, not sure why but so far so good.