IIS Application won't allow new webitems

This is absolutely bizarre! I have an IIS Application which I have compiled and it runs happily. There are several webitems, both HTML templates and custom webitems.

I now need to make a modification by adding a new webitem. BUT WHEN I DO, there is a call to a subroutine that simply STOPS executing! Single stepping through the code I hit the line:
*****************************************
If CheckLogin(Request.Form("txtUserName"), Request.Form("txtPassword")) Then
*****************************************

Well normally what this does is go into the CheckLogin function which returns True or False. But now when I add a new webitem, The code simply REFUSES to go into the subroutine! Single stepping (or F5)It just goes to the END IF without even checking! This is a persistant, repeatable problem, in that I can remove my new webitem, code works fine, add new webitem, the call doesn't work.

What's going on behind the scenes here that I don't understand?

Thanks