枝分かれの探索がうまく動作しない

前提・実現したいこと

参考ページ
上記のURLを参考にナンプレ(数独)を解くプログラムを作成しています。
上記のURLのコードをつなぎ合わせると確かに答えを導き出せることも確認できましたので、自分なりに書いてみようと思い同じようなコードを書いたのですが、探索の途中で探索が終了していまい、答えにたどりつくことができません。print()やステップ実行を使って原因を探ってみたのですが、自分の力では解決に至りませんでした。
探索が途中で終了してしまうのがなぜかを知りたいです。

該当のソースコード

import csv

def main():
    pass

def import_csv(filename):
    """
    csvファイルから問題の読み込み
    """
    with open(filename, 'r') as f:
        reader = csv.reader(f)
        rows = []
        for row in reader:
            for index, value in enumerate(row):
                if value == '':
                    row[index] = 0
                else:
                    row[index] = int(value)
            rows.append(row)
    return rows

def export_quiz(filename, quiz):
    """
    csvファイルへ出力
    """
    with open(filename, 'w') as f:
        writer = csv.writer(f, lineterminator='\n')
        for row in quiz:
            writer.writerow(row)

def display_quiz(quiz):
    """
    ナンプレ風に表示
    """
    for row_index, row in enumerate(quiz):
        for index, value in enumerate(row):
            if value == '':
                value = '0'
            if index == 8:
                print(value)
            else:
                print(value, end=' ')

            if index in (2, 5):
                print('|', end='')

        if row_index in (2, 5):
            print('------+------+------')

def solve(quiz, point=[0, 0]):
    """
    再帰的にナンプレを解く
    """
    # global count
    # count += 1
    # print(count)
    # export_quiz('quiz1_ans_count_' + str(count) + '.csv', quiz)

    if point[0] > 8:
        return True

    if quiz[point[0]][point[1]] != 0:
        if solve(quiz, next_point(point)):
            return True
    else:
        for number in range(1, 10):
            if check(quiz, point, number):
                quiz[point[0]][point[1]] = number
                if solve(quiz, next_point(point)):
                    return True
        quiz[point[0]][point[1]] = 0
        return False
    print('×')
    # display_quiz(quiz)

def check(quiz, point, number):
    """
    quizのpointにnumberを入れられるかを判定する
    return boolean
    """
    return all([check_row(quiz, point[0], number), check_column(quiz, point[1], number), check_block(quiz, point, number)])

def check_row(quiz, row_index, number):
    """
    quizのrow_indexにnumberを入れられるかを判定する
    """

    return number not in quiz[row_index]

def check_column(quiz, column_index, number):
    """
    quizのcolumn_indexにnumberを入れられるかを判定する
    """
    column = []
    for row_index in range(9):
        column.append(quiz[row_index][column_index])

    return number not in column

def check_block(quiz, point, number):
    """
    quizのpointにnumberを入れられるかを判定する
    """
    block = [] #同一ブロックに属する要素を抽出して格納する
    row_start = (point[0] // 3) * 3
    column_start = (point[1] // 3) * 3
    for row_index in range(row_start, row_start + 3):
        for column_index in range(column_start, column_start + 3):
            block.append(quiz[row_index][column_index])

    return number not in block

def next_point(point):
    """
    次の座標を返す
    [0, 0] → [0, 1] → ･･･ → [0, 8] → [1, 0] → ･･･
    point[0]: row
    point[1]: column
    """
    if point[1] == 8:
        point[0] += 1
        point[1] = 0
    else:
        point[1] += 1

    return point

if __name__ == '__main__':
    filename = 'quiz1'
    quiz = import_csv(filename + '.csv')
    export_quiz(filename + '_ans.csv', quiz)

    display_quiz(quiz)
    count = 0
    quiz = solve(quiz)
    # print(quiz)
    # display_quiz(quiz)

quiz1.csv

8,5,,,,2,4,,
,,2,,,,,,9
,,4,,,,,,
,,,1,,7,,,2
3,,5,,,,9,,
,4,,,,,,,
,,,,8,,,7,
,1,7,,,,,,
,,,,3,6,,4,

試したこと

関数のテストコードの実装,printデバッグ、ステップ実行デバッグ

import unittest
import number_place

class TestNumberPlace(unittest.TestCase):
    quiz = number_place.import_csv('quiz.csv')
    number_place.display_quiz(quiz)

    def test_next_point(self):
        self.assertEqual(number_place.next_point([0, 0]), [0, 1])
        self.assertEqual(number_place.next_point([0, 8]), [1, 0])
        self.assertEqual(number_place.next_point([2, 8]), [3, 0])
        self.assertEqual(number_place.next_point([8, 8]), [9, 0])

        # point = [0, 0]
        # for n in range(1, 101):
        #     point = number_place.next_point(point)
        #     print(point)

    def test_check_row(self):
        self.assertEqual(number_place.check_row(TestNumberPlace.quiz, 0, 1), False)
        self.assertEqual(number_place.check_row(TestNumberPlace.quiz, 0, 4), True)
        self.assertEqual(number_place.check_row(TestNumberPlace.quiz, 0, 7), False)
        self.assertEqual(number_place.check_row(TestNumberPlace.quiz, 1, 7), True)
        self.assertEqual(number_place.check_row(TestNumberPlace.quiz, 1, 1), True)

    def test_check_column(self):
        self.assertEqual(number_place.check_column(TestNumberPlace.quiz, 0, 1), True)
        self.assertEqual(number_place.check_column(TestNumberPlace.quiz, 0, 5), False)
        self.assertEqual(number_place.check_column(TestNumberPlace.quiz, 1, 9), True)
        self.assertEqual(number_place.check_column(TestNumberPlace.quiz, 1, 7), False)

    def test_check_block(self):
        self.assertEqual(number_place.check_block(TestNumberPlace.quiz, [3, 6], 1), True)
        self.assertEqual(number_place.check_block(TestNumberPlace.quiz, [3, 7], 1), True)
        self.assertEqual(number_place.check_block(TestNumberPlace.quiz, [3, 8], 1), True)
        self.assertEqual(number_place.check_block(TestNumberPlace.quiz, [4, 6], 1), True)
        self.assertEqual(number_place.check_block(TestNumberPlace.quiz, [4, 7], 1), True)
        self.assertEqual(number_place.check_block(TestNumberPlace.quiz, [4, 8], 1), True)
        self.assertEqual(number_place.check_block(TestNumberPlace.quiz, [5, 6], 1), True)
        self.assertEqual(number_place.check_block(TestNumberPlace.quiz, [5, 7], 1), True)
        self.assertEqual(number_place.check_block(TestNumberPlace.quiz, [5, 8], 1), True)

        self.assertEqual(number_place.check_block(TestNumberPlace.quiz, [5, 8], 2), False)

    def test_check(self):
        self.assertEqual(number_place.check(TestNumberPlace.quiz, [1, 0], 1), True)


if __name__ == '__main__':
    unittest.main()