There is a very nice GUI tool to troubeshoot webinject scripts. You can download the tool @ http://www.webinject.org/download.html for Windows and Linux.
After you extract or install the tool you can change the globalhttplog line in the config.xml file to: <globalhttplog>yes</globalhttplog> so you get the full debug log.
Note: I found a small memory bug in the GUI when you use the parseresponse parameter in your scripts. To workaround this issue the best you can do is restart the tool (file > restart) before you test the scripts.