How to add transposition table to Negamax?

Question

I've recently been improving my Negamax algorithm for chess, adding a transposition table. The algorithm was working well before this addition. After this addition I can't see no big improvements in speed, and sometimes I even see some regressions. I tested it with a depth of 5 and 6.

Is there any error in my code? (P.S.: tell me if the question is not clear or incomplete, I will edit it) Thank you!

Here's the code, the new pieces are the commented lines at the top and at the bottom:

private int negaMax(Board board, Move[] legalMoves, int depth, int turnMultiplier, int alpha, int beta) {

    //Search in transposition table
    /*int initialAlpha = alpha;
    int transpositionDepth = 0;
    String transpositionFlag = "";
    int transpositionEvaluation = 0;
    String fenString = board.GetFenString().Split(" ")[0];
    if (transpositionTable.ContainsKey(fenString)) {
        transpositionDepth = Int32.Parse(transpositionTable[fenString].Split(" ")[0]);
        transpositionFlag = transpositionTable[fenString].Split(" ")[1];
        transpositionEvaluation = Int32.Parse(transpositionTable[fenString].Split(" ")[2]);

        if(transpositionDepth >= depth) {
            if(transpositionFlag.Equals("Exact")) { return transpositionEvaluation; }
            else if (transpositionFlag.Equals("Lowerbound")) { alpha = Math.Max(alpha, transpositionEvaluation); }
            else if (transpositionFlag.Equals("Upperbound")) { beta = Math.Min(beta, transpositionEvaluation); }

            if(alpha >= beta) { return transpositionEvaluation; }

        }
    }*/

    //Base case
    if (depth == 0) { return evaluatePosition(board) * turnMultiplier; }

    //Recursion
    int bestEvaluation = -infinity;

    foreach(Move move in board.GetLegalMoves()) {

        board.MakeMove(move);
        legalMoves = board.GetLegalMoves();
        orderMoves(legalMoves);
        int evaluation = -negaMax(board, legalMoves, depth - 1, -turnMultiplier, -beta, -alpha);

        if(evaluation > bestEvaluation) {
            bestEvaluation = evaluation;
            if (depth == searchDepth) { bestMove = move; }
        }

        board.UndoMove(move);

        //Pruning
        alpha = Math.Max(alpha, bestEvaluation);
        if(alpha >= beta) {
            break;
        }
    }

    //Transposition table update
    /*transpositionEvaluation = bestEvaluation;
    if (bestEvaluation <= initialAlpha) { transpositionFlag = "Upperbound"; }
    else if (bestEvaluation >= beta) { transpositionFlag = "Lowerbound"; }
    else { transpositionFlag = "Exact"; }
    transpositionDepth = depth;
    if (!transpositionTable.ContainsKey(fenString)) {
        transpositionTable.Add(fenString, transpositionDepth + " " + transpositionFlag + " " + transpositionEvaluation);
    }*/

    return bestEvaluation;
}