C pipes with execlp(). Code works for "head" command but not for "sort"

So my job is to write a function that will work like:

ps -ef| tr -s ' '  :| cut -d: -f1 | sort | uniq -c |sort -n 

Here is my code:

void pipe4()
{
  int pdesk1[2];
  int pdesk2[2];
  int pdesk3[2];
  int pdesk4[2];
  int pdesk5[2];

  pipe(pdesk1);
  pipe(pdesk2);
  pipe(pdesk3);
  pipe(pdesk4);
  pipe(pdesk5);

  if (pdesk1 < 0 || pdesk2 < 0 || pdesk3 < 0 || pdesk4 < 0 || pdesk5 < 0)
  {
    perror("pipe");
    exit(1);
  }

  if (fork() == 0)
  {
    dup2(pdesk1[1], 1);
    close(pdesk1[0]);
    close(pdesk1[1]);
    execlp("ps", "ps", "-e", "-f", NULL);
    perror("ps");
    exit(1);
  }

  if (fork() == 0)
  {
    dup2(pdesk1[0], 0);
    dup2(pdesk2[1], 1);
    close(pdesk2[0]);
    close(pdesk2[1]);
    close(pdesk1[0]);
    close(pdesk1[1]);
    execlp("tr", "tr", "-s", " ", ":", NULL);
    perror("tr");
    exit(1);
  }
  if (fork() == 0)
  {
    dup2(pdesk2[0], 0);
    dup2(pdesk3[1], 1);
    close(pdesk3[0]);
    close(pdesk3[1]);
    close(pdesk2[0]);
    close(pdesk2[1]);
    execlp("cut", "cut", "-d", ":", "-f", "1", NULL);
    perror("cut");
    exit(1);
  }
  if (fork() == 0)
  {
    dup2(pdesk3[0], 0);
    // commented to see result at this moment
    // dup2(pdesk4[1], 1);
    close(pdesk4[0]);
    close(pdesk4[1]);
    close(pdesk3[0]);
    close(pdesk3[1]);
    //this prints nothing 
    execlp("sort", "sort", NULL);
    // but this works
    // execlp("head", "head", NULL);
    perror("sort");
    exit(1);
  }

  if (fork() == 0)
  {
    dup2(pdesk4[0], 0);
    dup2(pdesk5[1], 1);
    close(pdesk5[0]);
    close(pdesk5[1]);
    close(pdesk4[0]);
    close(pdesk4[1]);
    execlp("uniq", "uniq", "-c", NULL);
    perror("uniq");
    exit(1);
  }

  if (fork() == 0)
  {
    dup2(pdesk5[0], 0);
    close(pdesk5[0]);
    close(pdesk5[1]);
    execlp("sort", "sort", "-n", NULL);
    perror("sort -n");
    exit(1);
  }
  wait(NULL);
  exit(1);
}

So as I wrote in comments, up until sort part everything works fine, I checked outputs in each process and compared it with results I get by writing command in terminal.

I'm even more confused since I found out that when I replace sort with head (or cat for example), code works, uniq seems to work fine and then again last sort doesn't work.