为什么这个AJAX请求需要花费很长时间才能进行多次调用？

I make an AJAX request to a PHP script which returns a number from a text file that changes. This AJAX request should happen every 3 seconds. However, an AJAX request is made once and it doesn't return anything and firebug shows the AJAX GET request is still being made. After a few minutes it returns and produces a number. It should have made multiple calls but it only made one and it just came back with a final answer. I am struggling to figure out how this happened!? :(I make an AJAX request to a PHP script which re