Problem with gfortran

[Pietiläinen Ville]

Hi,
I have Dell T5500 desktop with 64 bit Windows 7, 12 GB RAM and two 12 core processors. I have fortran90 source code not written by me. As far as I know the programmer has 32 bit windows and is not familiar with my problems.
The code has many loops which are parallelized using OpenMP.

I compiled using gfortran (I have used many versions from different sources , but current one is 4.6 experimental).  I used command "c:\mingw644\bin\gfortran -fopenmp -m64 -O3 -march=native -fmax-stack-var-size=10000000 ." following with all the files. The program runs beautifully but it seems not to use all computational power there is. As I compiled using Intel Fortran, the code used all my 24 cores 100%, I checked this from task manager. But when I use gfortran version, it may be that 12 of the cores are "parked" doing nothing. Now I tried different input model which is bit larger and now it uses all 24 cores but none of which with full power. It  uses only 50-60% of total CPU. 

Is there something I am doing wrong with gfortran or is this just a difference between Intel Fortran and gfortran? I would really like to use gfortran but I would also like to use all computational power my desktop has.

Thank you very much

Best regards,
Ville Pietiläinen