what is wrong with my minmax with alpha beta pruning?

Question

when i apply minmax only to my connect4 game it get right results untill depth 7 and when i apply pruning to increase depth it get wrong results , i need a help to kno what is wrong with my logic or implementation?

private int score()
    {
        int winner = check_Winner(); // return 1 if player  win or 2 when pc win or 0
        if (winner == 1) return 10;
        else if (winner == 2) return -10;
        else return 0;
    } 
    public int MinMax(int player_num, int depth, int alpha, int beta)
    {
        List<pair> possiple_moves = new List<pair>();
        get_possible_moves( possiple_moves);
        //================================================
        int score_so_far = score();
        if (possiple_moves.Count == 0 || score_so_far != 0 || depth == 10)// terminal case
            return score_so_far;
        //================================================
        if (player_num == 1)// maxmize
        {
            int max_score = int.MinValue, temp = 0;
            foreach (pair move in possiple_moves)
            {
                cells[move.x, move.y].player_num = 1;

                temp = MinMax(2, depth + 1, alpha, beta);
                if (temp > max_score)
                {
                    max_score = temp;
                    pc_move = move;
                }

                cells[move.x, move.y].player_num = 0;
                alpha = Math.Max(alpha, max_score);
                if (beta <= alpha)
                    break;
            }
            return max_score;
        }
        //==========================
        else
        {
            int min_score = int.MaxValue, tmp = 0;
            foreach (pair move in possiple_moves)
            {
                cells[move.x, move.y].player_num = 2;

                tmp = MinMax(1, depth + 1, alpha, beta);
                if (tmp < min_score)
                {
                    min_score = tmp;
                    pc_move = move;
                }

                cells[move.x, move.y].player_num = 0;
                beta = Math.Min(beta, min_score);
                if (beta <= alpha)
                    break;
            }
            return min_score;
        }
    }