I'm only finding a 6 card deck

puzzle_solver.rb

require 'json'

class Array
    def rotate n
        a =dup
        n.times do a << a.shift end
        a
    end
end


@grid = [[{"type"=>"p", "head" => 1},{"type"=>"c", "head" => 1},{"type"=>"a", "head" => 2},{"type"=>"o", "head" => 2}],
[{"type"=>"o", "head" => 1},{"type"=>"a", "head" => 2},{"type"=>"c", "head" => 2},{"type"=>"p", "head" => 1}],
[{"type"=>"c", "head" => 1},{"type"=>"p", "head" => 2},{"type"=>"o", "head" => 2},{"type"=>"a", "head" => 1}],
[{"type"=>"p", "head" => 1},{"type"=>"c", "head" => 2},{"type"=>"o", "head" => 2},{"type"=>"a", "head" => 1}],
[{"type"=>"p", "head" => 2},{"type"=>"c", "head" => 2},{"type"=>"a", "head" => 1},{"type"=>"c", "head" => 1}],
[{"type"=>"a", "head" => 1},{"type"=>"p", "head" => 2},{"type"=>"o", "head" => 2},{"type"=>"p", "head" => 1}],
[{"type"=>"a", "head" => 1},{"type"=>"o", "head" => 1},{"type"=>"a", "head" => 2},{"type"=>"c", "head" => 2}],
[{"type"=>"o", "head" => 1},{"type"=>"a", "head" => 2},{"type"=>"c", "head" => 2},{"type"=>"p", "head" => 1}],
[{"type"=>"p", "head" => 1},{"type"=>"c", "head" => 2},{"type"=>"o", "head" => 2},{"type"=>"a", "head" => 1}]]
@new_grid = [nil, nil, nil,nil, nil, nil,nil, nil, nil]
@used = [false, false, false,false, false, false,false, false, false]

def check_validity(card, position, orientation)
    # since I'm adding from top left to bottom, I only need to check top and left           
    try_card = @grid[card].rotate orientation     
    valid = true
    # top
    if (@new_grid[position-3])
        if (try_card[0]["type"] != @new_grid[position-3][2]["type"] || try_card[0]["head"] == @new_grid[position-3][2]["head"])
            valid = false
        end
    end
    # left
    if (@new_grid[position-1] && (position % 3) != 0)
        if (try_card[3]["type"] != @new_grid[position-1][1]["type"] || try_card[3]["head"] == @new_grid[position-1][1]["head"])
            valid = false       
        end
    end
    return valid
end


def solve_puzzle(position)
    (0..8).each do |card|
        unless (@used[card])
            (0..3).each do |orientation|
                if (check_validity(card, position, orientation))
                    @used[card] = true
                    @new_grid[position] = @grid[card].rotate orientation
										if position == 7 
											puts @new_grid.to_json										
										end
                    if (position < 8)
                        solve_puzzle(position + 1)
                    else
                        puts "I WON"
                        puts @new_grid.to_json
                    end
                    @new_grid[position] = nil
                    @used[card] = false
                end
            end
        end
    end
end


solve_puzzle(0)