Very limited info online about this.
Its getting too be stupid. I put a brand new SHS tappetplate in, Same problem.,
Did you re-shim your gearbox or leave it as is?
I found this Vid on Youtube, The first half is the problem I have, Starting at 36sec. I don't have any misfeeding issues.
AND just too clear it up, I don't have that Ear splitting motor whine either LOL.
Airsoft AEG: Tappet Plate and Air Nozzle Troubleshoot - YouTube