finally I solve the problem by myself, as layman in php coding,
I have notice other files css & js are also inactive,
but plugins are working fine,
the button was not showing because of following, if you find
“single_add_to_cart_button button alt single_add_sample_to_cart_button btn btn-default” in woocommerce-sample.php
I only remove ending “btn btn-default”
and leave this “add_to_cart_button button alt single_add_sample_to_cart_button” in php file. the button is showing as per my theme button which add to cart
HO HO HO -I AM THE KING
