流水灯出现问题

初学Arduino，在练习流水灯的时候遇到了问题
const int Nbrleds = 6;
const int ledPins[] = {3,5,6,9,10,11};
const int wait = 35;
const int top = 40;
const int mid = 20;
const int bom = 10;
void setup(){
  for (int led = 0;led < Nbrleds;led++)
  {pinMode(ledPins[led],OUTPUT);}
}
void loop(){
for (int led = 0;led<Nbrleds - 1;led++)
{ analogWrite(ledPins[led],top);
delay(wait);
analogWrite(ledPins[led+1],top);
  analogWrite(ledPins[led],mid);
delay(wait);
  analogWrite(ledPins[led+2],top);
analogWrite(ledPins[led+1],mid);
  analogWrite(ledPins[led],bom);
  delay(wait);
digitalWrite(ledPins[led],LOW);
delay(wait*3);
}
for (int led = Nbrleds - 1;led > 0;led--)
{ analogWrite(ledPins[led],top);
delay(wait);
analogWrite(ledPins[led - 1],top);
  analogWrite(ledPins[led],mid);
delay(wait);
analogWrite(ledPins[led - 2],top);
analogWrite(ledPins[led - 1],mid);
  analogWrite(ledPins[led],bom);
  delay(wait);
digitalWrite(ledPins[led],LOW);
delay(wait*3);
}
}


想用pmw实现拖尾效果，期初程序运行没有问题，灯可以正常流动，大概10次之后10、9灯就不亮了。又过了几次循环，11、5、3灯变成了常亮。不知道哪里出现的问题，请各位大神指点，谢谢