Sorry for the posting the question.
I have solved the problem. Actually the residues names were not of standard type.
Bla
The slab representation of nucleic acid under Depiction is not working. I made the following settings
show backbone as atoms/bonds, show sugar as atoms/bonds, show base as slab. But the representation is not displayed on the screen.
What could be the reason. I myself have used it before and i am not aware why this happens now.
Regards,
Bala