The stock servos are absolutly terrible. U can try taking it apart and looking at the gears specificly the one thats had tge arm attached bcuz it has a peice on it that tells the servo if its straight or not.
Get a better servo, hitec 645mg is a good one. It has metal gears and like 160oz torque. I have one on my rs10 I have an exi 227 something. The hitec cost me 35 and the exi was 25. The hitec IMO is better. Nothing wrong with the exi though.