XML Encoder Serialization

Greetings.
I'm working on a design in which I had hoped to use XMLEncoder to
store objects to disk. While testing it on two of my classes, I
observed an inconsistency in which fields get added to the XML
document. I use code like the following to write them out:

<o gets declared/initialized>
String f = "c:\\tmp\\packet.xml";
XMLEncoder out = new XMLEncoder(new BufferedOutputStream(new
FileOutputStream(f)));
out.writeObject(o);
out.close();

When I create a class from scratch, this appears to write out every
member of o for which there is a getter and setter defined. However, I
tried doing it to two of my existing classes with varying results. The
class HBitField (see below) worked as expected, with all four member
fields being written to XML. Unfortunately HPacketEntry (below) only
prints the "name" and "numWords" fields -- omitting "wordSize" and
"bitFields". I have accessors defined for all of them, and the
protection level is the same. Why are these classes being treated
differently by the Encoder? Is there some caveat I'm missing here?

Any insight would be greatly appreciated.

#########################
HBitField
#########################
public class HBitField {

    protected int FirstWord;

    protected int StartBit;

    protected int Length;

    protected String Name;

    public HBitField() {
        super();
        FirstWord = 0;
        StartBit = 0;
        Length = 0;
        this.setName("");
    }
....
<other ctors, methods>
....

    public String getName() {
        return Name;
    }

    public void setName(String name) {
        Name = name;
    }

    public int getFirstWord() {
        return FirstWord;
    }

    public void setFirstWord(int firstWord) {
        FirstWord = firstWord;
    }

    public int getLength() {
        return Length;
    }

    public void setLength(int length) {
        Length = length;
    }

    public int getStartBit() {
        return StartBit;
    }

    public void setStartBit(int startBit) {
        StartBit = startBit;
    }
}

#########################
HPacketEntry
#########################
public class HPacketEntry {

    protected int wordSize;
    protected int numWords;
    protected String name;

    private HashSet bitFields;

    public HPacketEntry() {
        this.wordSize = 16;
        this.numWords = 1;
        this.bitFields = new HashSet();
        this.name = new String("default");
    }
....
<other ctors, methods>
....
    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public int getWordSize() {
        return wordSize;
    }

    public void setWordSize(int wordSize) {
        this.wordSize = wordSize;
    }

    public HashSet getBitFields() {
        return bitFields;
    }

    public void setBitFields(HashSet bitFields) {
        this.bitFields = bitFields;
    }

    public int getNumWords() {
        return numWords;
    }

    public void setNumWords(int numWords) {
        this.numWords = numWords;
    }
}