trouble incrementing variable

i having trouble cycling through shift register.  i'm looking individual output shift register high when pin 2 on arduino receiving  5v.  cycling through pins on 595 each button press.  have working initial value once value goes low again stops working.  suggestions appreciated.  here code:

code: [select]

#include <shifter.h>

#define ser_pin 8 //data
#define rclk_pin 9 //latch
#define srclk_pin 10 //clock

#define num_registers 1

const int buttonpin = 2;
int buttonstate = low;
int n = 0;

shifter shifter(ser_pin, rclk_pin, srclk_pin, num_registers);

void setup(){

}

void loop()
{
  shifter.clear();
  shifter.write();

buttonstate = digitalread(buttonpin);

if (buttonstate == high)
  {
  shifter.setpin(n, high);
  shifter.write();
  delay(150);
  n++;
  }
}

in setup make buttonpin input.

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