You are not passing all the graded tests. Before submitting your assignment, it is necessary to pass all the tests.
You are failing the conv_forward test. Your vert_start, vert_end, horiz_start, and horiz_end are wrong. You missed the stride factor. Also, your implementation of a_slice_prev is incorrect. Loop over i, then use vert_start, vert_end, horiz_start, horiz_end. Maybe this guide will help you.