Problem : 
If the page is first rendered with the calendar not showing, and then through ajax refresh you try to display it the calendar will not render.

Solution:

To fix this go inside the FullCalendar2 Module and search for the OSFullCalendar function definition. There you should have this code : 

SyntaxEditor Code Snippet

function OSFullCalendar (calendarPlaceHolderId, eventHandlerBlockId) {
    this.calendarPlaceHolderId = calendarPlaceHolderId;
    this.eventHandlerBlockId = eventHandlerBlockId;
    this.calendar = undefined;
    var instance = this;

    this.init = function (options) {
        var calendarEl = document.getElementById(calendarPlaceHolderId);
        if(window.hascalendar===undefined) { // to allow the use of Ajax Refresh 
            document.addEventListener('DOMContentLoaded', function() {
                window.hascalendar = true; 
                var calendarEl = document.getElementById(calendarPlaceHolderId); 
                var cal = new FullCalendar.Calendar(calendarEl, options);
                cal.render();
                instance.calendar = cal;
            }); 
        } else {                        // to allow the use of Ajax Refresh 

The problem is that for some reason that event that we binded the render logic is never triggering on an ajax refresh.

To fix this change to  :

SyntaxEditor Code Snippet

function OSFullCalendar (calendarPlaceHolderId, eventHandlerBlockId) {
    this.calendarPlaceHolderId = calendarPlaceHolderId;
    this.eventHandlerBlockId = eventHandlerBlockId;
    this.calendar = undefined;
    var instance = this;

    this.init = function (options) {
        var calendarEl = document.getElementById(calendarPlaceHolderId);
        if(window.hascalendar===undefined) { // to allow the use of Ajax Refresh   
            $("#"+calendarPlaceHolderId).ready(function() { 
                window.hascalendar = true; 
                var calendarEl = document.getElementById(calendarPlaceHolderId); 
                var cal = new FullCalendar.Calendar(calendarEl, options);
                cal.render();
                instance.calendar = cal;
            }); 
        } else {                        // to allow the use of Ajax Refresh 
            var calendarEl = document.getElementById(calendarPlaceHolderId);
            var cal = new FullCalendar.Calendar(calendarEl, options);
            cal.render();
            instance.calendar = cal;
        };
    }

Best Regards,

Pedro Neto