Selenium.click not working on some anchor elements

Try selenium.fireEvent(locater, 'click'), or using Selenium 2 which is more tightly integrated with the browser.

You may be having the same problem as some other people, eg.

Selenium clicks not working with GWT

Using Selenium to 'click' on non-input or non-control elements

It seems to be related to click events which are added with Javascript.

Edited

I don't know if you're using the same calendar implementation, but I discovered that the fullcalendar.js jQuery one replaces the mouseover event, and you have to trigger that first. I got it to work using

selenium.runScript("jQuery(\"a:contains('" + NEW_EVENT_NAME        + "')\").trigger('mouseover');jQuery(\"a:contains('"        + NEW_EVENT_NAME + "')\").trigger('click')");

Our JQuery calendar implementation is compatible with the default Selenium locators, even though the locators appear disabled in the DOM. Here's an example, for you to try:

selenium.click("link=11:00 AM - 01:00 PM");

Ok, I just assume your XPATH to get that span is correct, I suspect your selenium script is running faster than your page load, so add this function to wait until the page is loaded

waitForPageToLoad

Hope helps:)