MPI not running in parallel in a FORTRAN code

I am trying to install an OpenMPI on my Ubuntu (14.04) machine, and I thought that I had succeeded, because I can run codes with mpirun, but recently I have noticed that it's not truly running in parallel.

I installed openmpi with the following options:

./configure CXX=g++ CC=gcc F77=gfortran \
                           F90=gfortran  \
                            FC=gfortran   \
         --enable-mpi-f77 \
         --enable-mpi-f90  \
         --prefix=/opt/openmpi-1.6.5
make all
sudo make install

As I said, I have run a code ( not written by myself ) and it seemed to work in parallel, because I checked with top and it was running in several nodes.

But now I have written a simple FORTRAN code:

PROGRAM hello_MPI
  INCLUDE "mpif.h"
  INTEGER :: err, size, rank
  CALL MPI_INIT(err)
  IF (err /= MPI_SUCCESS) STOP 'Init failed'
  CALL MPI_COMM_RANK(MPI_COMM_WORLD, rank, err)
  CALL MPI_COMM_SIZE(MPI_COMM_WORLD, size, err)
  PRINT*, "Hello world from process ", rank, " of ", size, " processes"
  CALL MPI_FINALIZE(err)
END PROGRAM

But when I run it with

mpirun -n 4 ./hello_MPI

I get the same output 4 times, showing that it is just running the same single process in 4 different processors

 Hello world from process            0  of            1  processes
 Hello world from process            0  of            1  processes
 Hello world from process            0  of            1  processes
 Hello world from process            0  of            1  processes

I have run this same code in a different machine and I get the expected output:

 Hello world from process            0  of            4  processes
 Hello world from process            1  of            4  processes
 Hello world from process            2  of            4  processes
 Hello world from process            3  of            4  processes

I managed to solve it. It was quite silly, to be honest. For some reason I was not using the right OpenMPI installation (I have no idea which it was using). I recompiled using the right mpif90 and run it using the right mpirun and now I get the expected result. Sorry for the confusion, but I am quite new to this. And thank you for the help!