TMS570LS3137: ADC conversion, missing samples every 256 samples

I am using TMS570 Hercules Dev Kit.

I am fighting with ADC (one channel), data acquisition frequency is determined by RT (e.g. 1-50kHz) and FATfs library for data acquisition (SD card slot). Everything looked fine but than I started tests with sinewaves from generator.... and .... wait where are my samples? ;/

The problem is, every 256 samples (it is constant value and it is independent from acquisition frequency) some samples are missing. I do not have any other interrupts. I checked time of code inside my interrupt and the max value is 7,0us. (rtiGetCurrentTick = 70, with closk 10M).

Here is my example data:

and my code:

   if (notification == rtiNOTIFICATION_COMPARE0)
    {
		/* ADC conversion */
		adcStartConversion(adcREG1, adcGROUP1);
		while(!adcIsConversionComplete(adcREG1, adcGROUP1));

		adcGetData(adcREG1, adcGROUP1, adc_data_ptr); //Store conversion into ADC pointer
		value = (uint16_t)((*adc_data_ptr).value);

		// Convert uint16_t data into 2x BYTE
		dataBufferPtr[0] = (value >> 8) & 0xFF;
		dataBufferPtr[1] = value & 0xFF;

		/* Write file */
		f_write(&fsrc, &dataBuffer, bufferSize, &bw);
		bufferFIlledCount = bufferFIlledCount + 1;
		dataBufferPtr = &dataBuffer;


		/* Check if number of conversion reached number required */
		if (bufferFIlledCount == buffersToBeSaved)
		{
			/* Close the file */
			res = f_close(&fsrc);
		}         
    }

Where could be a problem? If it can help I can also add screenshots from my HalCoGen project.