Не могу найти логическую ошибку. C#, Игра - Connect Four

public class ConnectFour
    {
        public static bool check(string[][] myField, int a, int b)
        {
          string player = myField[a][b];
          int vertical = 1;
          int horizontal = 1;
          int diagonal1 = 1;
          int diagonal2 = 1;
          int i;//vertical
          int ii;//horizontal
          //check for vertical(|)
          i = a + 1;
          while(i<=5)//Check up
          {
            if(i == a + 1 && myField[i][b] != player) break;
            if(myField[i][b] == player) vertical++;
            i++;
          }
          i = a - 1;
          while(i >= 0)//Check down
          {
            if(i == a - 1 && myField[i][b] != player) break;
            if(myField[i][b] == player) vertical++;
            i--;
          }
          if(vertical >= 4)return true;
          //check for horizontal(-)
          ii = b - 1;
          while(ii>=0) //Check left
          {
            if(ii == b - 1 && myField[a][ii] != player) break;
            if(myField[a][ii] == player) horizontal++;
            ii--;
          }
          ii = b + 1;
          while(ii <=6) //Check right
          {
            if(ii == b + 1 && myField[a][ii] != player) break;
            if(myField[a][ii] == player) horizontal++;
            ii++;
          }
          if(horizontal >= 4) return true;
        //check for diagonal 1 (\)
        i = a - 1;
        ii = b - 1;
        while(i>=0 && ii >=0) //up and left
        {
          if(i == a - 1 && ii == b - 1 && myField[i][ii] != player) break;
          if(myField[i][ii] == player) diagonal1++;
          i--;
          ii--;
        }
        i = a + 1;
        ii = b + 1;
        while(i<=5 && ii <=6) //down and right
        {
          if(i == a + 1 && ii == b + 1 && myField[i][ii] != player) break;
          if(myField[i][ii] == player) diagonal1++; 
          i++;
          ii++;
        }
        if(diagonal1 >= 4) return true;
        //check for diagonal 2(/)
        i = a - 1;
        ii = b + 1;
        while(i>=0 && ii <= 6) //up and right
        {
          if(i == a - 1 && ii == b + 1 && myField[i][ii] != player) break;
          if(myField[i][ii] == player) diagonal2++;
          i--;
          ii++;
        }
        i = a + 1;
        ii = b - 1;
        while(i<=5 && ii >=0) //up and left
        {
          if(i == a + 1 && ii == b - 1 && myField[i][ii] != player) break;
          if(myField[i][ii] == player) diagonal2++;
          i++;
          ii--;
        }
        if(diagonal2 >= 4) return true;

          return false;
        }
        public static string WhoIsWinner(List<string> piecesPositionList)
        {
           int a1 = 0;
           int b1 = 0;
           int c1 = 0;
           int d1 = 0;
           int e1 = 0;
           int f1 = 0;
           int g1 = 0;
           string temp = ""; 
           string result = "";
           string[][] myField = new string[6][];
           for(int i = 0; i < 6; i++) myField[i] = new string[7];
           for(int i = 0; i < piecesPositionList.Count; i++)
           {
               temp = piecesPositionList[i];
               if(temp[0] == 'A')
               {
                  myField[a1][0] = temp.Substring(2);
                  if(check(myField, a1, 0)) return temp.Substring(2);
                  ++a1;
               }
               if(temp[0] == 'B')
               {
                 myField[b1][1] = temp.Substring(2);
                 if(check(myField, b1, 1)) return temp.Substring(2);
                 ++b1;
               }
               if(temp[0] == 'C')
               {
                 myField[c1][2] = temp.Substring(2);
                 if(check(myField, c1, 2)) return temp.Substring(2);
                 ++c1;
               }
               if(temp[0] == 'D')
               {
                 myField[d1][3] = temp.Substring(2);
                 if(check(myField, d1, 3)) return temp.Substring(2);
                 ++d1;
               }
               if(temp[0] == 'E')
               {
                 myField[e1][4] = temp.Substring(2);
                 if(check(myField, e1, 4)) return temp.Substring(2);
                 ++e1;
               }
               if(temp[0] == 'F')
               {
                 myField[f1][5] = temp.Substring(2);
                 if(check(myField, f1, 5)) return temp.Substring(2);
                 ++f1;
               }
               if(temp[0] == 'G')
               {
                 myField[g1][6] = temp.Substring(2);
                 if(check(myField, g1, 6)) return temp.Substring(2);
                 ++g1;
               }
           }
           return "Draw";
        }
    }