arduino - Servo not maintaining desire "zero" position controlling it with a PCA9685 driver -

i building arm in each finger controlled servo. using adafruit pca9685 servo driver interface servos. control script running on arduino. trying figure out proper way control each servo move 0 180 degrees close hand , 180 0 open. servos using towar pro mg946r. when have servo under no tension @ tendons or string pull on fingers, move ok 0 180 degrees, when attach strings 1 servo , close , open hand few times servo losses 0 position (it goes passed 0 position or doesn't there , 180 degrees there messing whole system up). important me servo no matter goes 0 position. please ask me many questions want if need me explained myself better. take @ code , recommend see convenient. thanks!!

    #include <wire.h>     #include <adafruit_pwmservodriver.h>      #define servomin 130     #define servomax 520      adafruit_pwmservodriver pwm = adafruit_pwmservodriver();      uint8_t thumb = 0;     uint8_t ring = 1;      void setup() {        serial.begin(9600);        serial.begin("servo test");         pwm.begin();         pwm.setpwmfreq(60);  // analog servos run @ ~60 hz updates         yield();     }      void loop() {      pwm.setpwm(ring, 0, servomin);      delay(1000);     //  (uint16_t pulselen = servomin; pulselen < servomax; pulselen++)                  {     //    pwm.setpwm(thumb, 0, pulselen);    //  }    //    //  delay(500);    //  (uint16_t pulselen = servomax; pulselen > servomin; pulselen--) {     //    pwm.setpwm(thumb, 0, pulselen);    //  }       pwm.setpwm(ring, 0, servomax);       delay(1000);       pwm.setpwm(ring, 0, servomin);       delay(1000);    }