having trouble using heuristic dissector

Hello,
I'm new to ethereal development and needed some help.  I have created a
heuristic dissector and associated it with all udp packets as follows:

// in the proto_reg_handoff _myproto, after checking initialized
// and creating the proto handle
heur_dissector_add("udp", heur_dissect_myproto, proto_myproto);

// The dissector looks like the following
// the previous ports:
static guint32 previous_src_port = -1;
static guint32 previous_dst_port = -1;
static gboolean heur_dissect_myproto(tvbuff_t *tvb, packet_info *pinfo,
                                                        proto_tree *tree)
{
    gshort myproto_head;
    guint8 myproto_packet_type;
    gint offset;

    offset = 0;
    // Check if there is really any data to parse!
    if (tvb_bytes_exist(tvb, 0, tvb_length_remaining(tvb, 0)) == FALSE)
        return FALSE;

    if (tvb_length_remaining(tvb, 0) < 5)
        return FALSE;

    // Check for the magic string and the type before parsing further
    myproto_head = tvb_get_ntohs(tvb, offset);

    if (myproto_head == MY_PROTO_HEAD)
        offset += 2;

    myproto_packet_type = tvb_get_guint8(tvb, offset);
    if (myproto_head == MY_PROTO_HEAD && myproto_packet_type <= 9)
    {
        previous_src_port = pinfo->srcport;
        previous_dst_port = pinfo->destport;
        call_dissector(myproto_handle, tvb, pinfo, tree);
        return TRUE;
    }
    else if (previous_src_port == pinfo->destport &&
             previous_dst_port == pinfo->srcport)
    {
        // I was never able to get the subtree working in the UI
        // with my original dissector.
        // however any packet that had the MY_PROTO_HEAD worked.
        // I guess during detailed information the static vars
        // get set to -1.  That might be why the detailed info
        // didn't show.
        previous_src_port = -1;         previous_dst_port = -1;
        call_dissector(myproto_handle, tvb, pinfo, tree);
        return TRUE;
    }

    return FALSE;

}

My question is why doesn't detailed information show for the else if
statement?  Should I just save the port off and add it as a filter, i.e.
dissector_add("udp.port", pinfo->destport, myproto_handle)?

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