CAPTCHA problem

I have a working PHPRunner 5.1 application that has been running well so far. However, today I noticed that the CAPTCHA has stopped producing a visible code. It is activated by doing a screen refresh in the browser. There was a fix for this published in this forum :
Go to C:\Program Files\PHPRunner5.1\source (or wherever you installed PHPRunner to) and open securitycode.php with your favorite text editor. At the top of the page you will see 3 lines like the following:
header("Expires: Thu, 01 Jan 1970 00:00:01 GMT");

header("Pragma: no-cache");

header("Cache-Control: no-cache");
You need to comment these out by placing "//" before each one like this:
//header("Expires: Thu, 01 Jan 1970 00:00:01 GMT");

//header("Pragma: no-cache");

//header("Cache-Control: no-cache");
However, i did this some time ago and this got it working originally. I can see that it has not changed. Any ideas as to why CAPTCHA has stopped updating properly?