RecyclerView仅显示顶部数据

Chat class

聊天课

private void fetchMessages() {
    rootRef.child("Messages").child(MessageSenderId).child(MessageRecieverId)
            .addChildEventListener(new ChildEventListener() {
        @Override
        public void onChildAdded(DataSnapshot dataSnapshot, String s) {
            Messages messages = dataSnapshot.getValue(Messages.class);
            messagesList.add(messages);
            messageAdapter.notifyDataSetChanged();
        }

        @Override
        public void onChildChanged(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onChildRemoved(DataSnapshot dataSnapshot) {

        }

        @Override
        public void onChildMoved(DataSnapshot dataSnapshot, String s) {

        }

        @Override
        public void onCancelled(DatabaseError databaseError) {

        }
    });
}
private void fetchMessages()