Why does my recursive backtracking stop at seemingly random points

Question

I am attempting to generate all closed curves in a finite region of the simple hexagonal lattice. This isn't too important, it is just a finite set of points a distance of 1 away from each other. My code however, will generate closed curves for a while, and then my program will quit working and get stuck in a infinite loop? I have tried forcing java to garbage collect by command, but the same code stops at different points. As far as I can tell, where it stops is random. Sphere is an array storing all the points in the region under consideration

private static Sphere Sphere1 = new Sphere();
private static double [][] vectors =  {{0, 0, 1},{0, 0, -1},{1, 0, 0},{-1,0,0},{.5, (Math.sqrt(3))/2, 0},{-.5, -(Math.sqrt(3))/2, 0},{.5, -(Math.sqrt(3))/2, 0},{-.5, (Math.sqrt(3))/2, 0}};
private static StringBuilder loopString = new StringBuilder("r");
private static String posPoints = "abcdefghijklmnopqrstuvwxyz1234567";
private static int garbage = 0;

private static boolean once = false;
private static PrintWriter output = null;

public static void main (String [] args){

    try {
        output = new PrintWriter(new FileOutputStream("closedLoops.txt"));
    } catch (FileNotFoundException e){
        System.out.println("error");
    }
    System.out.println("start");
    KnotPoint loopPoint = new KnotPoint(0,0,1);
    addVector(loopPoint, vectors);

}

public static void addVector(KnotPoint loopPoint, double [][] vectors){
    garbage ++;
    if (garbage == 200){
        System.gc();
        garbage = 0;
    }
    if (loopString.length() > 19
       && (loopString.charAt(loopString.length()-1) == 'z' ||
           loopString.charAt(loopString.length()-1) == '3' ||
           loopString.charAt(loopString.length()-1) == 'u' ||
           loopString.charAt(loopString.length()-1) == 'o' ||
           loopString.charAt(loopString.length()-1) == 'g' ||
           loopString.charAt(loopString.length()-1) == 'j' ||
           loopString.charAt(loopString.length()-1) == 'q')) 
    {
        System.out.println(loopString);
        output.println(loopString);
        once = true;

        return;
    }
    for (int i = 0; i < 8 ; i ++){
        if (validAdd(loopPoint, vectors[i], loopString, posPoints)){
            loopPoint.addNext(vectors[i]);

            addVector(loopPoint, vectors);
            //System.gc();
            loopString.deleteCharAt(loopString.length()-1);
            loopPoint.subtractLast(vectors[i]);
            if(loopString.toString().equals("r") ){
                output.println("you did it");
                output.close();
                System.out.println("you did it good job");
                System.exit(0);
            }
        }
    }

    return;
}

public static boolean validAdd(KnotPoint loopPoint, double [] vector, StringBuilder loopString, String posPoints){
    KnotPoint testAdd = new KnotPoint();
    testAdd.set(loopPoint);
    testAdd.addNext(vector);
    int pointIndex = 0;
    boolean pointCheck = false;
    char point = '.';
    for (int i = 0; i < 33; i ++){
        if( testAdd.equals(Sphere1.getKnotPoint(i))){
            pointIndex = i;
            pointCheck = true;
            point = posPoints.charAt(i);
        }
    }
    if (pointCheck && !loopString.toString().contains(posPoints.substring(pointIndex, pointIndex + 1))&& point != 'r'){// added not r check
        loopString.append(point);
        return true;
    } else {
        return false;
    }
}