On click function only working once

On click function only working once

Hello,
 
I'm new to jquery and have created this calendar using jquery/ajax/php:
 
 
When you click the forward (>>) and/or back (<<) buttons, they work, but only the once - not sure what I'm missing?  Here is the code if you need to see that:
 
$(document).ready(function(){
  $("#next_month").click(function(){
  //alert('hi next month!');
   var month = $("#next_month_value").text();
   if ($("#next_year_value").text()) var nextyear = $("#next_year_value").text();
   var string = "m="+month;
   if (nextyear) string += "&y="+ nextyear;
      $.ajax({  
   type: "POST",  
   url: "events_calendar.php",  
   data: string,
  success: function(data) {
  //alert(data);
         $('#events_calendar').html(data);
       } 
   });  
  });
  $("#last_month").click(function(){
  //alert('hi last month!');
   var month = $("#last_month_value").text();
   if ($("#last_year_value").text()) var lastyear = $("#last_year_value").text();
   var string = "m="+month;
   if (lastyear) string += "&y="+ lastyear;
      $.ajax({  
   type: "POST",  
   url: "events_calendar.php",  
   data: string,
  success: function(data) {
  //alert(data);
         $('#events_calendar').html(data);
       } 
   });  
  });
});

































Thanks in anticipation.
 
Cybril