Automated parallelizing not working with large matrixes

I have a code in fortran that calculates eigenvectors and eigenfunctions of matrix and process the results. I use the ifort with the -mkl compilation, the code runs in parallel for an square matrix (N,N) when N is less than 100 (more or less) but when I try larger matrixes N>300 it only runs as one thread. 

Any help?

Regards

Eric