N-Queens Problem: My Code Doesn't Work With n = 5, Sometimes Other n Too...

I have an n sized int array containing a permutation of 0...n. I use this to conceptualize an nXn chessboard, using the array index i for the rows of the board, and array[i] as the columns. So for a permutation of n = 4, I have, say, (2,1,0,3) , giving me a board with queens at (0,2),(1,1),(2,0),(3,3). This ensures that no two rows have a queen in the same column. I then call optimize() on this permutation, which checks whether there are attacks on the board (totalAttacks()). This total includes attack for queens a and b, for a to b and b to a, so a pair of attacking queens adds 2 to the count. numAttacks(int i) counts all attacks by the queen in the ith row, not including itself. swap(int i, int j) simply swaps the two elements at the given indices. so swap(0, 1) on the given permutation above gives me (1,2,0,3). fill() gives a new permutation if the current permutation cannot be optimized in this way. Below is the code for the optimize() method. It works pretty well for n < 15, but occasioinally hangs for n > 12, and always hangs for n = 5. Why does it hang for n = 5, why does it sometimes hang for other n, and what can I change to get this to work for n = 5 and larger (maybe up to n = 20)? Thanks in advance for your help -supr
        public void optimize() {
                int iathen, ianow, jathen, janow;
                while(totalAttacks() != 0) {
                        for(int i = 0; i < n; i++) {
                                for(int j = i + 1; j < n; j++) {
                                        iathen = numAttacks(i);
                                        jathen = numAttacks(j);
                                        if(iathen != 0 && jathen != 0) {
                                                swap(i, j);
                                                ianow = numAttacks(i);
                                                janow = numAttacks(j);
                                                if(iathen < ianow && jathen < janow)
                                                        swap(i, j);
                                                }

                                        }
                                if(i == n - 1)
                                        fill();
                                }
                        }
                }