Electronic – Low Pass IIR filter attenuating all frequencies

I am trying to make a Low pass IIR filter in dsPIC(dsPIC33FJ12MC202) using mikroC compiler. I have generated filter coefficients using Filter Design Tool. My filter specifications are as follows:
LPF, Wp=4KHz, Ws=5KHz, Ap=0, As=60, Sampling about 20Khz

I am taking analog input from channel zero, filtering it and sending filtered value via UART. The code look something like this:

int adcValue;
char result[6];

const unsigned int BUFFER_SIZE   = 8;
const unsigned int FILTER_ORDER  = 3;

const unsigned int COEFF_B[FILTER_ORDER+1] = {0x17D9, 0x478B, 0x478B, 0x17D9};
const unsigned int COEFF_A[FILTER_ORDER+1] = {0x8000, 0xAF27, 0x383C, 0xF802};

const unsigned int SCALE_B = 1;
const unsigned int SCALE_A = 0;

unsigned int inext;
ydata unsigned int input[BUFFER_SIZE];
ydata unsigned int output[BUFFER_SIZE];

void filter(unsigned int adcValue)
{
  unsigned int CurrentValue;
  input[inext] = adcValue;

  CurrentValue = IIR_Radix( SCALE_B,
                            SCALE_A,
                            COEFF_B,        // b coefficients of the filter
                            COEFF_A,        // a coefficients of the filter
                            FILTER_ORDER+1, // Filter order + 1
                            input,          // Input buffer
                            BUFFER_SIZE,    // Input buffer length
                            output,         // Output buffer
                            inext);         // Current sample

  //CurrentValue = 2048;

  output[inext] = CurrentValue;
  inext = (inext+1) & (BUFFER_SIZE-1);    // inext = (inext + 1) mod BUFFER_SIZE;

  //Sending filtered value via UART
  WordToStr(CurrentValue, result);
  strcat(result, "\n\r");
  UART1_Write_Text(result);
}


void main()
{  
   inext  = 0;                                 // Initialize buffer index
   Vector_Set(input, BUFFER_SIZE, 0);         // Clear input buffer
   Vector_Set(output, BUFFER_SIZE, 0);       // Clear output buffer

   //Using R12, R13 for Rx, Tx
   PPS_Mapping(12, _INPUT, _U1RX);
   PPS_Mapping(13, _OUTPUT, _U1TX);

   UART1_Init(115200);
   ADC1_Init();

   while(1)
   {
      adcValue = ADC1_Read(0);
      Delay_us(50);
      filter(adcValue);
   }
}

The IIR_Radix function is provided by mikroC compiler, So I am expecting below 4Khz it should give output whatever the input is and above 4KHz it should attenuates input.

Currently I am simulating circuit using Proteus and when I apply Sine Wave generator(Amplitude 5v, Freq=1000 Hz) at ADC channel I am getting output which has maximum value 95(e.g 0, 22, 69, 95, 69, 22, 0…) on Virtual Terminal. But 1000 Hz is in passband so I should get whatever the input is, like(0, 25, 499, 1023, 499, 25, 0…). So why I am getting attenuated output even for passband frequencies?

I am analyzing output on virtual terminal inside proteus simulation, see on right.