MPU-6500 FIFO is not filling up

I would like to read MPU-6500 3axis accelerometer values from FIFO, using SPI, at 4kHz ODR, lowpass filters disabled, but it looks like buffer starts with ~72 samples and does not get filled up. Can anyone look at the snippets of my code?

function init_MPU6500()      //make stetting to MPU-6500

{

spi_write_reg(PWR_MGMT_1,0x80)    // Rst internal regs and restores the default settings

imp.sleep(0.1)                    // recommended wait time by datasheet

spi_write_reg(SIGNAL_PATH_RST,0x07)// recommended by datasheet

imp.sleep(0.1)                    // recommended wait time by datasheet

spi_write_reg(ACCEL_CONFIG2,0x08) // set ODR to 4kHz and bypass digital low pass filter (BW=1.13kHz)

spi_write_reg(FIFO_EN,0x08)       // write accelerometer data to FIFO

spi_write_reg(USER_CTRL,0x50)     // enable FIFO operation, SPI only mode

spi_write_reg(PWR_MGMT_2,0x07)    // enabel accels, disable gyros

imp.sleep(0.1)                    // recommended wait time by datasheet

}



function loop_MPU6500()

{   //to display the g's on the screen

spi_read_reg(FIFO_COUNT_H,2)            // these registers contain the number of bytes in fifo

local bytes_in_fifo=data[0]<<8|data[1]  // OR the high and low byte

if(bytes_in_fifo>5){                    // if there are 6 bytes in fifo we can display them

spi_read_reg(FIFO_R_W,6)              // read 6 bytes from fifo

local x = data[0]<<8|data[1]          // left shift high byte and OR with low bye

local y = data[2]<<8|data[3]          // left shift high byte and OR with low bye

local z = data[4]<<8|data[5]          // left shift high byte and OR with low bye

if(x&0x8000){x=-((~x&0x7fff)+1)}      // to convert two's complement to a float

if(y&0x8000){y=-((~y&0x7fff)+1)}      // if the bit15 is set

if(z&0x8000){z=-((~z&0x7fff)+1)}      // invert the bits and add 1

x=x*61e-6       // each digit is 4g/2^16=61ug

y=y*61e-6       // each digit is 4g/2^16=61ug

z=z*61e-6       // each digit is 4g/2^16=61ug

server.log(format("%i, X:%0.6fg, Y: %0.6fg, Z: %0.6fg",bytes_in_fifo,x,y,z))

}

imp.wakeup(0.3, loop_MPU6500)  // this loop repeats every x second

}



init_MPU6500()  // Set up accelerometer

loop_MPU6500()

2014-12-28 13:59:17 UTC-8	[Device]	66, X:0.080520g, Y: -0.023424g, Z: 0.992104g

2014-12-28 13:59:16 UTC-8	[Device]	72, X:0.080520g, Y: -0.018056g, Z: 0.995032g

2014-12-28 13:59:17 UTC-8	[Device]	60, X:0.080520g, Y: -0.023424g, Z: 0.992104g

2014-12-28 13:59:17 UTC-8	[Device]	54, X:0.081984g, Y: -0.017568g, Z: 1.000888g

2014-12-28 13:59:18 UTC-8	[Device]	48, X:0.081984g, Y: -0.017568g, Z: 1.000888g

2014-12-28 13:59:18 UTC-8	[Device]	42, X:0.075152g, Y: -0.020008g, Z: 0.997472g

2014-12-28 13:59:18 UTC-8	[Device]	36, X:0.075152g, Y: -0.020008g, Z: 0.997472g

2014-12-28 13:59:19 UTC-8	[Device]	30, X:0.072224g, Y: -0.019520g, Z: 0.995520g

2014-12-28 13:59:19 UTC-8	[Device]	24, X:0.072224g, Y: -0.019520g, Z: 0.995520g

2014-12-28 13:59:19 UTC-8	[Device]	18, X:0.073688g, Y: -0.023424g, Z: 1.007720g

2014-12-28 13:59:20 UTC-8	[Device]	12, X:0.073688g, Y: -0.023424g, Z: 1.007720g

2014-12-28 13:59:20 UTC-8	[Device]	6, X:0.081984g, Y: -0.030744g, Z: 1.003816g