If you strike a metal stake in a stump, a good way togrind is to have oxy cutter on site. It will burn it deeper than angle grinder. grind down to stake and then burn deeper again.
Anvil should be square and sharp. not rounded off.
Set clearance to hacksaw blade, 1/32".
Keep the revs up!
Make sure there are no obstructions in the chute, bends, daggy weld etc.
hope this helps.
I have repaired one of these. Take out the rotor, bore a hole the same dia as the shaft 6" deep, cut 2 1/2" key ways full depth. Have a shaft made to press in and accept the keys. Much cheaper than a new rotor.