ProcessMessage not called after certain time of calls

I am new to this smack api.I tried to develop a client using smack api from the tutors available online. It worked fine for me but after certain number of chats between two clients the processmessage method is not called.

Can anyone help me regarding this plz....

code goes here.

public void sendMessage(String message, String to) throws XMPPException

    {

    Chat chat = connection.getChatManager().createChat(to, this);

    chat.sendMessage(message);

    }  

    public void processMessage(Chat chat, Message message)

    {

    if(message.getType() == Message.Type.chat)

    System.out.println(chat.getParticipant() + " says: " + message.getBody());

    }

Thanks in advance