why is the if condition not being executed even if it is true? [closed]

why does 'if(mat[i][j] == 0)' not execute?

for i in range(9) :
        for j in range(9) :

            if (mat[i][j] == 0) :

                print(i , " " , j , " FOUND EMPTY")

                for n in range(1,10) :

                    if (checkNum(i,j,n)) :

                        mat[i][j] = n

                        if (placeNum()) :
                            return True

                        else :
                            mat[i][j] = 0

                return False

    #sudoku solved
    return True