AVR Freaks

I need a working debounce code for a single push button. This push button I use common shaft rotary encode type which consist of a Push Button.  It is not a momentary switch.  Pls don't ask me to use momentary switch as I need to use this encoder type.

I use the below code which is not working properly. The error is it the Mode jump from 1,2,0, 1,2,0 ..It doesn't go into Mode 3. Even I change the debounce to 6 states, it is still working.  

Any ideas to fix this from the below code. 

void debounce(void){
	//Counter for number of equal states
	static uint8_t count = 0;
	//Keep track of current states
	static uint8_t button_state = 0;
	
	//Check if button is high or low for the moment
	uint8_t current_state = (~PINA & BUTTON_MASK) !=0;
	
	if (current_state != button_state){
		//button state is about to be change
		count++;
		if (count >=4){
			//The button have not bounced for four state, change state
			button_state = current_state;
			//if the button was pressed, tell me so
			if (current_state !=0) {
				button_down =1;
				
			}
			count =0;
		}
		} else {
		count = 0;
	}
	
}






int main(){
    

	while (1)
	{
        debounce();			
	    
        if (button_down)
           {
	           button_down = 0;//reset flag
	       Mode ++;
	        //disp.setNumber(mode);
	       if (Mode==4)  Mode = 0;
	     	switch (Mode)
	         {
			case 0 ://nothing
	         bla....
	         
	         }
           }
	}
}