Problem in subgridding method in FDTD

Hi to all.
is there someone that is familiar with FDTD and particular Subgridding methods?
I have one problem.
I wrote a subgridding program for 3D-fdtd.But there is a problem.
when in the program,I remove fine mesh effect over coarse mesh,my result is convergent,always.
But when fine mesh effect over coarse mesh exist,my answers become divergent.
does anyone know that why this problem is occured?
also,I should note that in my program,I used from PML bondary conditions.
I tested same program with Yee's FDTD method ( full fine mesh),and in this case,my answers are true.
please help me...
my email : shaban_sadeghi@yahoo.com
best regards...
ghasem