Trying to write to end of file, but overwriting instead

Question

Im using a microblaze soft processor on a Basys3 dev board. I am able to write a text file to an SD card but I am trying to write to the end of the file. Currently it overwrites anything that's in the file. Im using the DFATFS functions to do the writing. The Basys3 has a PMOD uSD reader attached.

I've tried using the fslseek function in DFATFS to move the file pointer to the end of file but it still writes to the beginning.

//the subroutine to print to the SD card

void SD(int32_t rando, int addr, int bit, int pc) {
DXSPISDVOL disk(XPAR_PMODSD_0_AXI_LITE_SPI_BASEADDR,
         XPAR_PMODSD_0_AXI_LITE_SDCS_BASEADDR);
   DFILE file;
   int eof;
   char printline[128];
   sprintf(printline, "\nLFSR: %d ", rando);
   // The drive to mount the SD volume to.
   // Options are: "0:", "1:", "2:", "3:", "4:"
   static const char szDriveNbr[] = "0:";

   // Mount the disk
   DFATFS::fsmount(disk, szDriveNbr, 1);

   xil_printf("Disk mounted\r\n");

   fr = file.fsopen("output.txt", FA_WRITE | FA_OPEN_ALWAYS);

   if (fr == FR_OK) {
      file.fslseek(0);
      file.fswrite(printline, 12, &bytesWritten);

      fr = file.fsclose();

   } else {
      xil_printf("Failed to open file to write to\r\n");
   }

}

I expect the code to move the file pointer to the end of the file and print a new line after the previous one. When stepping through the program (multiple times through the print subroutine) but it always overwrites.